A cold thing (not hot) on stage (6)
I believe the answer is:
acting
'on stage' is the definition.
(I've seen this before)
'a cold thing not hot' is the wordplay.
'cold' becomes 'c' (eg on taps).
'not' indicates named letters should be taken away.
'hot' becomes 'h' (abbreviation, eg on taps).
'thing' with 'h' removed is 'ting'.
'a'+'c'+'ting'='ACTING'
(Other definitions for acting that I've seen before include "Temporary - work in the theatre" , "Behaving like thespian" , "Temporary - work as a thespian" , "Interim" , "Feigning" .)