Bunch going topless for mass (4)
I believe the answer is:
lump
'for mass' is the definition.
(mass can mean a lump or amount of something)
'bunch going topless' is the wordplay.
'bunch' becomes 'clump' (synonyms).
'going topless' means to remove the first letter.
'clump' with its first letter taken away is 'LUMP'.
(Other definitions for lump that I've seen before include "Irregularly shaped piece" , "Small shapeless mass" , "Swelling under the skin" , "Large piece without definite shape" , "Swelling; ungainly person" .)