Attacking cooking on gas that's cheap (5,3,1,4)
I believe the answer is:
going for a song
'cheap' is the definition.
(I've seen this before)
'attacking cooking on gas' is the wordplay.
'attacking' becomes 'going for' (to go for someone is to attack them).
'cooking' indicates an anagram (cook can mean to falsify or distort).
'on'+'gas'='ongas'
'ongas' anagrammed gives 'asong'.
'goingfor'+'asong'='GOING FOR A SONG'
'that's' is the link.
(Other definitions for going for a song that I've seen before include "Available at a bargain price" , "Available very cheap" , "Being offered very cheaply" , "Being sold very cheaply" , "Selling very cheaply" .)