Unwind (5)
'unwind' is the definition.
(I know that unwind can be written as relax)
This is the entire clue.
(Other definitions for relax that I've seen before include "Slow down and unwind" , "Slacken" , "Wind down" , "Become less tense" , "Loosen up, slow down" .)
(Other definitions for chillax that I've seen before include "Rest from work" , "Calm down and take it easy (informal)" , "Calm down and relax (informal)" .)