Straightens (7)
I believe the answer is:
unbends
'straightens' is the definition.
(I know this)
This is the entire clue.
(Other definitions for unbends that I've seen before include "Becomes less formal - straightens up" , "Becomes relaxed or affable" , "Straightens back to become less formal" , "grows warmer" , "Relaxes from tension" .)