Fit Mike fools around (5)
I believe the answer is:
spasm
'fit' is the definition.
(I've seen this in another clue)
'mike fools around' is the wordplay.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'fools' becomes 'saps' ('sap' can be a synonym of 'fool').
'around' says the letters should be written in reverse.
'm'+'saps'='msaps'
'msaps' back-to-front is 'SPASM'.
(Other definitions for spasm that I've seen before include "Muscle twitch" , "Convulsion or paroxysm" , "Tic, sudden contraction" , "Convulsive action" , "Jerk" .)