Excuse me, answer what upset Mike (4)
I believe the answer is:
ahem
'excuse me' is the definition.
(I've seen this before)
'answer what upset mike' is the wordplay.
'answer' becomes 'a' (abbreviation eg in Q and A).
'what' becomes 'eh' (I've seen this in other clues).
'upset' says the letters should be written in reverse.
'mike' becomes 'm' (phonetic alphabet: alpha, bravo, charlie etc.).
'eh' backwards is 'he'.
'a'+'he'+'m'='AHEM'
(Other definitions for ahem that I've seen before include "Half-cough to draw attention" , "Attention-grabbing expression" , "Polite word to draw attention" , "Noise made when clearing throat (4" , "Throat clearance to attract attention" .)