A bit of trifle and a little biter (4)
I believe the answer is:
flea
'little biter' is the definition.
(I've seen this in another clue)
'a bit of trifle and a' is the wordplay.
'a bit of' indicates a hidden word.
'FLEA' is hidden in the letters of 'trifle and a'.
(Other definitions for flea that I've seen before include "Blood-sucking parasites" , "Jumping, blood-sucking insect" , "Small parasitic insect" , "Leaping insects" , "little pest" .)