A jolly poster, perhaps, introducing a naval force (6)
I believe the answer is:
armada
'naval force' is the definition.
(I've seen this in another clue)
'a jolly poster perhaps introducing a' is the wordplay.
'jolly' becomes 'RM' (jolly can mean a Royal Marine).
'poster perhaps' becomes 'ad' (I can't justify this - if you can you should give a lot more credence to this answer).
'introducing' says to put letters next to each other (some letters go before others).
'a'+'rm'+'ad'+'a'='ARMADA'
Can you help me to learn more?
(Other definitions for armada that I've seen before include "Fleet of armed ships" , "Fighting unit" , "Large flotilla" , "Spanish fleet sent in 1588 to escort an invasion of England" , "vessels" .)