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' means one lot of letters goes inside another.
'mouth' becomes 'gob' (informal term for the mouth).
'l' going into 'gob' is 'GLOB'.
(Other definitions for glob that I've seen before include "Semi-liquid lump" , "Viscous lump" .)