Fighting half-heartedly, being merciful (7)
I believe the answer is:
sparing
'merciful' is the definition.
(I've seen this in another clue)
'fighting half-heartedly' is the wordplay.
'fighting' becomes 'sparring' (to spar is to box).
'half' suggests halving the letters.
'heartedly' becomes 'ing' (I can't explain this - if you can you should give a lot more credence to this answer).
'sparring' cut in half is 'spar'.
'spar'+'ing'='SPARING'
'being' acts as a link.
Can you help me to learn more?
(Other definitions for sparing that I've seen before include "Thrifty" , "mean" , "Economical; having mercy" , "Scanty - merciful" , "Moderate, economical" .)