Desire strongly - yes, to join the Navy! (5)
I believe the answer is:
yearn
'desire strongly' is the definition.
(I've seen this in another clue)
'yes to join the navy' is the wordplay.
'yes' becomes 'yea' (I've seen this before).
'to join' says to put letters next to each other.
'the navy' becomes 'RN' (Royal Navy).
'yea'+'rn'='YEARN'
(Other definitions for yearn that I've seen before include "wish" , "Long - pine" , "Desire, want" , "Pant - pine" , "lust" .)