Unwind (5)
I believe the answer is:
relax
'unwind' is the definition.
(I know that unwind can be written as relax)
This is all the 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" .)