What may be stuffed in banger, say, before function (5)
I believe the answer is:
cargo
'what may be stuffed in' is the definition.
(cargo might be stuffed into a vehicle)
'banger say before function' is the wordplay.
'banger say' becomes 'car' (a banger is an old car).
'before' says to put letters next to each other.
'function' becomes 'go' (synonyms).
'car'+'go'='CARGO'
(Other definitions for cargo that I've seen before include "Goods carried on ship or plane" , "Consignment" , "Freight on board ship" , "Stuff on board" , "Payload" .)