Stun gun initially held by old actor (7)
I believe the answer is:
stagger
'stun' is the definition.
(thesaurus)
'gun initially held by old actor' is the wordplay.
'initially' says to take the initial letters.
'held by' is an insertion indicator.
'old actor' becomes 'stager' (archaic term for an actor).
The first letter of 'gun' is 'g'.
'g' put inside 'stager' is 'STAGGER'.
(Other definitions for stagger that I've seen before include "Reel - astound" , "Reel about; shock" , "move by someone under the table?" , "Totter" , "Reel; amaze" .)