Animal's temperature in helium perhaps going up (4)
I believe the answer is:
stag
'animal's' is the definition.
(stag is a kind of animal)
'temperature in helium perhaps going up' is the wordplay.
'temperature' becomes 't' (abbreviation).
'in' is an insertion indicator.
'helium perhaps' becomes 'gas' (helium is a kind of gas).
'going up' is a reversal indicator.
't' going within 'gas' is 'gats'.
'gats' reversed gives 'STAG'.
(Other definitions for stag that I've seen before include "No women allowed" , "One who speculates" , "Stockmarket operator gambling on a quick profit" , "Antlered beast" , "Men-only" .)