Close hostilities with ultimatum, finally (4)
I believe the answer is:
warm
'close' is the definition.
(similar in meaning)
'hostilities with ultimatum finally' is the wordplay.
'hostilities' becomes 'war' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'finally' indicates one should take the final letters.
The last letter of 'ultimatum' is 'm'.
'war'+'m'='WARM'
(Other definitions for warm that I've seen before include "Lively" , "Affectionate - balmy" , "showing enthusiasm" , "Affectionate; heat" , "Moderately hot" .)