Michelangelo might turn in undercover agent (4)
I believe the answer is:
mole
'undercover agent' is the definition.
(mole is a kind of undercover agent)
'michelangelo might turn in' is the wordplay.
'in' indicates a hidden word.
'turn' says the letters should be written in reverse.
'elom' is hidden in the letters of 'michelangelo might'.
'elom' written backwards gives 'MOLE'.
(Other definitions for mole that I've seen before include "digger" , "Groyne" , "Adrian ---, fancies Pandora; lurking spy" , "Spy; burrower" , "beauty spot" .)