Go off with a gun, returning for timepiece (9)
I believe the answer is:
regulator
'timepiece' is the definition.
(device for regulating a clock's speed)
'go off with a gun returning' is the wordplay.
'go off with' becomes 'rota' (I can't justify this - if you can you should believe this answer much more).
'a gun' becomes 'luger' (type of pistol).
'returning' says the letters should be written in reverse.
'rota'+'luger'='rotaluger'
'rotaluger' reversed gives 'REGULATOR'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for regulator that I've seen before include "Ofsted perhaps" , "Such as OFGAS, OFTEL etc" , "Supervisor" , "Controller" , "part of the watch" .)