Straighten up and relax (6)
I believe the answer is:
unbend
I believe this clue is a double definition.
'straighten up' is the first definition.
('unbend' can be a synonym of 'straighten up')
'relax' is the second definition.
('unbend' can be a synonym of 'relax')
'and' is the link.
(Other definitions for unbend that I've seen before include "Become less reserved" , "Relax - straighten out!" , "Cease to stand on one's dignity" , "Take (exam) again" , "Relax, become more affable" .)