Mourn grouse after odd parts plucked (3)
I believe the answer is:
rue
'mourn' is the definition.
(thesaurus)
'grouse after odd parts plucked' is the wordplay.
'after odd parts plucked' means one should take alternating letters (letters in odd-numbered positions plucked out).
The alternate letters of 'grouse' are 'RUE'.
(Other definitions for rue that I've seen before include "Wish not to have been" , "Feel regret for the herb" , "At one time, pity" , "Be regretful" , "Nice way" .)