Go out of shape permanently (3)
I believe the answer is:
'go' is the definition.
(I've seen this before)
'out of shape permanently' is the wordplay.
'out of' says the answer is hidden in the clue.
'PEP' can be found hidden inside 'shape permanently'.
(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" .)