Go out of shape, possibly (3)
I believe the answer is:
pep
'go' is the definition.
(I've seen this in another clue)
'out of shape possibly' is the wordplay.
'out of' indicates a hidden word.
'PEP' is hidden within 'shape possibly'.
(Other definitions for pep that I've seen before include "Palindrome for vitality or energy" , "Energy, zip" , "Vim and vitality" , "Ginger; tax-free investment" , "Go" .)