The 23-year-old will leave Boreham Wood for Crawley Town on a one-year-deal.
Billy Clifford signed at the age of 8 for Chelsea and started to play in 2010 professional. In January 2013 he went for one month on loan to Colchester.
He spent last season at Boreham Wood and made 21 appearances.
Crawley's coach Dermot Drummy said:
"I know Billy from my time at Chelsea and I am aware what he is capable of and the qualities he has. He can play in the system we prefer and has lots of ability.
"This is a great chance for him. I have been impressed with him over the last few weeks and I think he can be a part of what we are trying to achieve at the club".