Wilt, using gas (3)
I believe the answer is:
sag
'wilt' is the definition.
(thesaurus)
'using gas' is the wordplay.
'using' indicates an anagram.
'gas' anagrammed gives 'SAG'.
(Other definitions for sag that I've seen before include "Droop in the middle - it's gas" , "Sinking" , "Sink (under load)" , "Cave in" , "Droop, flag" .)