Performing animal one is holding (2,5)
I believe the answer is:
on stage
'performing' is the definition.
(I've seen this before)
'animal one is holding' is the wordplay.
'animal' becomes 'stag' (stag is a kind of animal).
'is holding' indicates putting letters inside.
'stag' placed within 'one' is 'ON-STAGE'.
(Other definitions for on stage that I've seen before include "playing a part?" , "visible to audience" , "like passengers in Wild West" , "Performing" , "in a play?" .)