A fighting man I go to join up (6)
I believe the answer is:
solder
'to join up' is the definition.
(I've seen this before)
'a fighting man i' is the wordplay.
I cannot really see how this works, but
'man' could be 'sol' and 'sol' is found in the answer.
The remaining letters 'der' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'go' is the link.
Can you help me to learn more?
(Other definitions for solder that I've seen before include "Fusible jointing alloy" , "Metal bond" , "Fusible alloy for joining metals" , "Join two metal surfaces, using an alloy" , "Join two metals with a melted alloy" .)