Mass of grass beginning to smell like that, gather bundles up (7)
I believe the answer is:
tussock
'mass of grass' is the definition.
'beginning to smell like that gather bundles up' is the wordplay.
'beginning to' indicates taking the first letters.
'like that' becomes 'so' (I've seen this in other clues).
'gather' becomes 'tuck' (synonyms).
'bundles up' indicates putting letters inside.
The first letter of 'smell' is 's'.
's'+'so'='sso'
'sso' going into 'tuck' is 'TUSSOCK'.
(Other definitions for tussock that I've seen before include "Bushy clump of grass" , "(Of grass) cut" , "Thick patch of grass" , "Dense clump of grass - stuck so (anag)" , "Grassy clump" .)