Gas starts to oxidise metal, affecting main supply (7)
I believe the answer is:
ammonia
'gas' is the definition.
(I've seen this before)
'starts to oxidise metal affecting main supply' is the wordplay.
'starts to' indicates taking the first letters.
'supply' is an anagram indicator (in a supple way).
The first letters of 'oxidise metal affecting' is 'oma'.
'oma'+'main'='omamain'
'omamain' with letters rearranged gives 'AMMONIA'.
(Other definitions for ammonia that I've seen before include "Pungent cleaning fluid" , "Pungent gas used in cleaning liquids" , "Gas compounded of nitrogen and hydrogen, NH3" , "Smelling salts (NH3)" , "Powerful cleaner" .)