Long for agreement on service (5)
I believe the answer is:
yearn
'long for' is the definition.
(I've seen this in another clue)
'agreement on service' is the wordplay.
'agreement' becomes 'yea' (archaic form of 'yes').
'on' says to put letters next to each other (in a down clue, letters appear on others).
'service' becomes 'RN' (abbreviation for Royal Navy).
'yea'+'rn'='YEARN'
(Other definitions for yearn that I've seen before include "Have a great longing" , "Long - pine" , "Mark of injury" , "lust" , "Long (for something)" .)