Grumble - this apple is sour (4)
I believe the answer is:
crab
This clue is a triple definition!
'grumble' is the first definition.
(I've seen this in another clue)
'this apple' is the second definition.
(crab apple is a kind of apple)
'is sour' is the third definition.
(thesaurus)
(Other definitions for crab that I've seen before include "Nautical nipper" , "move obliquely" , "One fired from gun" , "Clawed shellfish" , "Decapod crustacean" .)