Discerning head of trade should stop moaning (8)
I believe the answer is:
sighting
'discerning' is the definition.
(sight can mean to discern or distinguish)
'head of trade should stop moaning' is the wordplay.
'head of' suggests taking the first letters.
'should stop' is an insertion indicator (stop can mean to plug a gap).
'moaning' becomes 'sighing'.
The initial letter of 'trade' is 't'.
't' going inside 'sighing' is 'SIGHTING'.
(Other definitions for sighting that I've seen before include "observation" , "Catching glimpse of" , "Managing to observe briefly" , "Managing to see" , "Spotting, espying" .)