Go out of shape, permanently (3)
I believe the answer is:
pep
'go' is the definition.
(both can mean energy or vitality)
'of shape permanently' is the wordplay.
'of' says the answer is hidden in the clue.
'PEP' is hidden within 'shape permanently'.
'out' is the link.
(Other definitions for pep that I've seen before include "Palindrome for vitality or energy" , "Go" , "savings plan" , "Ginger; tax-free investment" , "Energy or enthusiasm" .)