Stenographer holds back gas (5)
I believe the answer is:
argon
'gas' is the definition.
(thesaurus)
'stenographer holds back' is the wordplay.
'holds' indicates the answer is hidden within the clue.
'back' says the letters should be written backwards.
'nogra' is hidden within 'stenographer'.
'nogra' in reverse letter order is 'ARGON'.
(Other definitions for argon that I've seen before include "An insert gas to groan about" , "Noble gas, atomic number 18" , "Chemical element" , "Inert eighteen ac gas" , "rare gas" .)