Massive sweater dismissing a smelly problem (5)
I believe the answer is:
jumbo
'massive' is the definition.
(synonyms)
'sweater dismissing a smelly problem' is the wordplay.
'sweater' becomes 'jumper' (I've seen this before).
'dismissing' indicates named letters should be taken away.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'smelly problem' becomes 'BO' (body odour).
'jumper' with 'per' taken away is 'jum'.
'jum'+'bo'='JUMBO'
(Other definitions for jumbo that I've seen before include "great" , "Owner of trunk" , "Extra large size" , "Huge and bulky" , "Big jet" .)