Indeed, service is long! (5)
I believe the answer is:
yearn
'long' is the definition.
(synonyms)
'indeed service' is the wordplay.
'indeed' becomes 'yea' (I've seen this before).
'service' becomes 'RN' (abbreviation for Royal Navy).
'yea'+'rn'='YEARN'
'is' is the link.
(Other definitions for yearn that I've seen before include "Feel desire" , "Have an intense feeling of longing" , "Long - pine" , "lust" , "Long (for something)" .)