Some grouse, raw, an underling served up inadvertently (8)
I believe the answer is:
unawares
'inadvertently' is the definition.
(thesaurus)
'some grouse raw an underling served up' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'served up' shows that the letters should be reversed in order (in a down clue, letters go up).
'serawanu' can be found hidden inside 'grouse raw an underling'.
'serawanu' backwards is 'UNAWARES'.
(Other definitions for unawares that I've seen before include "New auras may be unexpected" , "without realising" , "without intending to" , "Off guard" , "Warn Esau (anag)" .)