Only one letter offering gas (6)
I believe the answer is:
butane
'gas' is the definition.
(I know that butane is a type of gas)
'only one letter' is the wordplay.
'only' becomes 'but' (but can mean only - eg 'life is but a dream').
'one' becomes 'an' (an apple is one apple).
'letter' becomes 'e'.
'but'+'an'+'e'='BUTANE'
'offering' is the link.
(Other definitions for butane that I've seen before include "Lighter fuel" , "Flammable gas in petroleum" , "Inflammable gas produced from petrol" , "Bottled gas used as a fuel" .)