Gas jet he regulated partly (5)
I believe the answer is:
ether
'gas' is the definition.
(thesaurus)
'jet he regulated partly' is the wordplay.
'partly' says the answer is hidden in the clue (part of the clue is used).
'ETHER' is hidden within 'jet he regulated'.
(Other definitions for ether that I've seen before include "Some were put out by this" , "Gas - air - space" , "The gas is there, oddly" , "Medium supposedly" , "Anaesthetic liquid; clear sky" .)