Endlessly pant for air (3)
I believe the answer is:
gas
'air' is the definition.
(thesaurus)
'endlessly pant' is the wordplay.
'endlessly' means to remove the last letter.
'pant' becomes 'gasp' (synonyms).
'gasp' with its final letter taken off is 'GAS'.
'for' acts as a link.
(Other definitions for gas that I've seen before include "American fuel" , "eg CO2" , "Eg, nitrogen" , "Fumes, vapour" , "Chatter (colloq.)" .)