I’m less than impressed with mammoth trade (3,4)
I believe the answer is:
big deal
'i'm less than impressed' is the definition.
(phrase used to show you're unimpressed)
'with mammoth trade' is the wordplay.
'with' means one lot of letters go next to another.
'mammoth' becomes 'big' (similar in meaning).
'trade' becomes 'deal' (synonyms).
'big'+'deal' is 'BIG DEAL'.
(Other definitions for big deal that I've seen before include "sarcastic response" , "Scornful response to boast (colloq.)" , "disparaging reply" , "Significant agreement - so what!" , "Exclamation of scorn, belittling a claim" .)