Breather? Gas fitter needs one at the start (7)
I believe the answer is:
inhaler
'breather?' is the definition.
(device to help someone breathe)
'gas fitter needs one at the start' is the wordplay.
'gas' becomes 'N' (chemical symbol for nitrogen).
'fitter' becomes 'haler' (hale can mean fit and healthy).
'needs one' becomes 'i' (Roman numeral. I am not sure about the 'needs' bit.).
'at the start' is a charade indicator (letters next to each other).
'n'+'haler'='nhaler'
'nhaler' after 'i' is 'INHALER'.
(Other definitions for inhaler that I've seen before include "gas mask" , "Essential for an asthma sufferer" , "Device for breathing in medication" , "Device for breathing in drug dose" , "item of medical equipment" .)