Born in Slough on 18 October 1992, and having come through the age groups in the Chelsea Academy, Billy can operate at right-back, in midfield, or wide right of the attack.
Competitive by nature, he played five times for the youth team when still a schoolboy. In his first full-time year he was outstanding as an attacking right-back right the way through to the 2010 FA Youth Cup Final win.
He was second highest youth appearance maker in 2009/10 and was given his reserve debut.
Having turned professional in July 2010, Billy started the first team's pre-season friendly away to Ajax, and celebrated his 18th birthday by joining the first team for a Champions League game in Moscow. He was an unused sub. He was also on the bench for the Champions League home game versus MSK Zilina in November 2010.
Injury prevented him from taking part in the later stages of the FA Youth Cup run to the semi-final or the reserves' league play-off success in the 2010/11 campaign, but he played 13 of the 21 matches that secured the national title at reserve team level and featured in the opening first team pre-season game ahead of 2011/12, against Wycombe Wanderers.
He then travelled to Asia in the summer of 2011 for the club's pre-season tour and enjoyed his first minutes of action with the senior side in wins against the Malaysian XI in Kuala Lumpur and the Thai Premier League All Stars in Bangkok.
Billy signed a new four-year contract in September 2011 before going on to feature regularly for the reserves throughout the ensuing campaign.
He made more appearances than any other player during the course of the 2011/12 season, turning out 19 times for Dermot Drummy's side and scoring two goals. He was also a member of the 10-man squad which travelled to take part in the Hong Kong Sevens tournament a few weeks after the season had finished.
He scored his first goal of the 2012/13 campaign in a 3-1 defeat away at Fulham in October, and was ever-present for the Under-21s prior to a move to League One Colchester on a month's loan on 11 January 2013.
That move was then extended until the end of the season in February, when after five appearances Billy had helped stem a run of nine straight defeats as Colchester battled against relegation into League Two.
His loan spell saw Clifford flourish, and the midfielder scored the first professional goal of his career with a 25-yard wonder strike against Yeovil.
His first game back at Chelsea saw him score the only goal of the game for our Under-21s in the 1-0 win over Blackburn Rovers.