Trim, in good shape, set up for the fight and taking one's shot (6)
I believe the answer is:
firing
'shot' is the definition.
(firing is a kind of shot)
'trim in good' is the wordplay.
'trim' becomes 'fir' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'good' becomes 'g' (abbreviation).
'fir'+'in'+'g'='FIRING'
'shape set up for the fight and taking one's' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for firing that I've seen before include "Discharging, sacking" , "Shooting; sacking" , "Dismissal" , "Discharging a gun" , "Dismissing" .)