Does battle losing head and part of a rifle (9)
I believe the answer is:
hindsight
'part of a rifle' is the definition.
(hindsight can mean the backsight of a rifle)
'does battle losing head' is the wordplay.
'does' becomes 'hinds' (hind can mean a female deer, a doe).
'battle' becomes 'fight' (fight is a kind of battle).
'losing head' means to remove the first letter.
'fight' with its initial letter removed is 'ight'.
'hinds'+'ight'='HINDSIGHT'
'and' acts as a link.
(Other definitions for hindsight that I've seen before include "Retrospection" , "Wisdom after the event" , "Thinking back" , "acquired too late to be useful?" , "Ability to understand, perhaps too late" .)