A jolly girl in navy (6)
I believe the answer is:
armada
'navy' is the definition.
(I've seen this before)
'a jolly girl' is the wordplay.
'jolly' becomes 'rm'.
'girl' becomes 'ada'.
'a'+'rm'+'ada'='ARMADA'
'in' acts as a link.
(Other definitions for armada that I've seen before include "Navy" , "Spanish fleet sent in 1588 to escort an invasion of England" , "sea-borne unit" , "A great fleet - Spain had one" , "force of the sea" .)