Mike leaving service, the idiot (3)
I believe the answer is:
ass
'idiot' is the definition.
(both can mean a stupid person)
'mike leaving service' is the wordplay.
'mike' becomes 'M' (phonetic alphabet: alpha, bravo, charlie etc.).
'leaving' suggests deleting specific letters.
'service' becomes 'mass' (church service).
'mass' with 'm' taken away is 'ASS'.
'the' acts as a link.
(Other definitions for ass that I've seen before include "Braying creature" , "Hoofed animal, smaller than a horse" , "sap" , "Stupid fellow" , "Pompous person" .)