A revolting expression of indifference used to attract attention (4)
I believe the answer is:
ahem
'used to attract attention' is the definition.
(you might cough to get someone's attention)
'a revolting expression of indifference' is the wordplay.
'revolting' is a reversal indicator.
'expression of indifference' becomes 'meh' ('meh' is an expression of indifference).
'meh' in reverse letter order is 'hem'.
'a'+'hem'='AHEM'
(Other definitions for ahem that I've seen before include "Utterance of mild disapproval" , "Sound used to attract attention" , "It attracts attention" , "Cough to draw attention" , "Excuse me!" .)