Causes some heat by writing on fighting (5)

I believe the answer is:
warms
'causes some heat' is the definition.
Although both the answer and definition are verbs in their -s form, I can't see how one could define the other.
'writing on fighting' is the wordplay.
'writing' becomes 'ms' (abbreviation for manuscript).
'on' means one lot of letters go next to another.
'fighting' becomes 'war'.
'ms' after 'war' is 'WARMS'.
'by' acts as a link.
Can you help me to learn more?
I've seen this clue in the Evening Standard.
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