Crow 'Welcome into service!'
I believe the answer is:
raven
'crow' is the definition.
(I have seen 'Large crow' mean 'raven' so perhaps 'crow' could also mean 'raven')
'welcome into service' is the wordplay.
'welcome' becomes 'Ave' (I've seen this before).
'into' means one lot of letters goes inside another.
'service' becomes 'RN' (abbreviation for Royal Navy).
'ave' put into 'rn' is 'RAVEN'.
(Other definitions for raven that I've seen before include "Crow-like bird" , "Flighty type" , "pig" , "Voraciously eat" , "Flyer" .)