Dollop left in mouth (4)
I believe the answer is:
glob
'dollop' is the definition.
(I've seen this in another clue)
'left in mouth' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'mouth' becomes 'gob' (informal term for the mouth).
'l' inserted within 'gob' is 'GLOB'.
(Other definitions for glob that I've seen before include "Semi-liquid lump" , "Viscous lump" .)