Bulge in pants Mrs Grundy conceals (8)
I believe the answer is:
protrude
'bulge' is the definition.
(synonyms)
'pants mrs grundy conceals' is the wordplay.
'pants' becomes 'rot' (both can mean rubbish or nonsense).
'mrs grundy' becomes 'prude' (Mrs Grundy can mean a prudish person).
'conceals' means one lot of letters goes inside another.
'rot' going inside 'prude' is 'PROTRUDE'.
'in' acts as a link.
(Other definitions for protrude that I've seen before include "Stick out, project from a surface" , "Stick outwards" , "Get through" , "Jut out" , "Stand out" .)