I leave a fighting man to form a union (6)
I believe the answer is:
solder
'i leave a fighting' is the definition.
I can't tell whether this definition defines the answer.
'man to form a union' is the wordplay.
'man' becomes 'Sol'.
'to form a union' becomes 'der' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sol'+'der'='SOLDER'
Can you help me to learn more?
(Other definitions for solder that I've seen before include "Join or fuse metal, using an alloy" , "Join two metal surfaces, using an alloy" , "Fusible alloy for joining metals" , "Join two metals with a melted alloy" , "Join two metal surfaces together" .)