The war at sea tended to hot up! (6)
I believe the answer is:
warmed
'tended to hot up' is the definition.
I can't tell whether this defines the answer.
'the war at sea' is the wordplay.
'at' means one lot of letters go next to another.
'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 "given thermal treatment" , "gave enthusiasm" , "Heated" .)