Long for agreement on service (5)
I believe the answer is:
yearn
'long for' is the definition.
(I've seen this before)
'agreement on service' is the wordplay.
'agreement' becomes 'yea' (I've seen this before).
'on' means one lot of letters go next to another.
'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)" .)