Feel a need, yes, for service (5)
I believe the answer is:
yearn
'feel a need yes for service' is the definition.
I can't judge whether this defines the answer.
'yes for service' is the wordplay.
'yes' becomes 'yea' (I've seen this before).
'for' means one lot of letters go next to another.
'service' becomes 'RN' (abbreviation for Royal Navy).
'yea'+'rn'='YEARN'
Can you help me to learn more?
(Other definitions for yearn that I've seen before include "wish" , "Hanker" , "Pant - pine" , "Long - pine" , "Have a great longing" .)