Long, yes, to join the navy (5)
I believe the answer is:
yearn
'long' is the definition.
('yearn' can be a synonym of 'long')
'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' (abbreviation for Royal Navy).
'yea'+'rn'='YEARN'
(Other definitions for yearn that I've seen before include "Pant - pine" , "Ache, crave" , "Desire strongly" , "Have strong desire" , "Curdle" .)