ITV say Detective Charlie McCluskey's retiring (7)
I believe the answer is:
channel
'itv say' is the definition.
(ITV is a TV channel)
'detective charlie mccluskey's retiring' is the wordplay.
'detective charlie' becomes 'chan' (Charlie Chan is a fictional detective).
'mccluskey' becomes 'len' (trade unionist Len McCluskey).
'retiring' is a reversal indicator.
'len' in reverse letter order is 'nel'.
'chan'+'nel'='CHANNEL'
(Other definitions for channel that I've seen before include "BBC1, perhaps" , "Underground route to France" , "Gutter; medium" , "Navigable part of a waterway" , "Water passage; TV station" .)