Trading insults finally before beer (4)
I believe the answer is:
sale
'trading' is the definition.
(thesaurus)
'insults finally before beer' is the wordplay.
'finally' suggests the final letters.
'before' means one lot of letters go next to another.
'beer' becomes 'ale' (type of beer).
The last letter of 'insults' is 's'.
's'+'ale'='SALE'
(Other definitions for sale that I've seen before include "Bargain-price event" , "Auction - somewhere in Greater Manchester" , "Cut-price offer" , "See 12 Down" , "Disposal - auction" .)