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