Converted triremes: most jolly! (8)
I believe the answer is:
merriest
'most jolly' is the definition.
('merry' can be a synonym of 'jolly')
'converted triremes' is the wordplay.
'converted' is an anagram indicator.
'triremes' is an anagram of 'MERRIEST'.
(Other definitions for merriest that I've seen before include "most tipsy" , "Most jolly and high spirited" , "Most mirthful" , "Jolliest" , "higher than the rest" .)