The war at sea hotted up! (6)
I believe the answer is:
warmed
'hotted up' is the definition.
The definition suggests a past participle verb which matches the answer.
'the war at sea' is the wordplay.
'at' is a charade indicator (letters next to each other).
'sea' becomes 'med' (abbreviation of Mediterranean).
'war'+'med'='WARMED'
Can you help me to learn more?
(Other definitions for warmed that I've seen before include "gave enthusiasm" , "Heated" , "given thermal treatment" .)