Unfair wager in which he blew lot stupidly (5,3,4)
I believe the answer is:
below the belt
'unfair' is the definition.
(I've seen this before)
'wager in which he blew lot stupidly' is the wordplay.
'wager' becomes 'bet' ('bet' can be a synonym of 'wager').
'in which' indicates putting letters inside.
'stupidly' indicates anagramming the letters.
'he'+'blew'+'lot'='heblewlot'
'heblewlot' is an anagram of 'lowthebel'.
'bet' enclosing 'lowthebel' is 'BELOW THE BELT'.
(Other definitions for below the belt that I've seen before include "Unsporting" , "Underhand, unfair" , "fly is here" , "Foul landing place for blow" , "Where unfair blow lands" .)