Left avoiding smooth build up (6)
I believe the answer is:
fatten
'build up' is the definition.
(thesaurus)
'left avoiding smooth' is the wordplay.
'left' becomes 'l' (common abbreviation).
'avoiding' is a deletion indicator.
'smooth' becomes 'flatten' (I've seen this before).
'flatten' with 'l' removed is 'FATTEN'.
(Other definitions for fatten that I've seen before include "Make plump" , "Make chubbier" .)