Becomes less tense and relaxes (7)
I believe the answer is:
unwinds
'becomes less tense and relaxes' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for unwinds that I've seen before include "Relaxes; pays out (thread)" , "Rests and becomes less tense" , "Becomes less tense and relaxes" .)