I’m at rear of Barlinnie, welcoming new prisoner (6)
I believe the answer is:
inmate
'prisoner' is the definition.
(inmate is a kind of prisoner)
'i'm at rear of barlinnie welcoming new' is the wordplay.
'rear of' suggests the final letters.
'welcoming' means one lot of letters goes inside another (in sense of 'inviting in').
'new' becomes 'n'.
The final letter of 'barlinnie' is 'e'.
'im'+'at'+'e'='imate'
'imate' going around 'n' is 'INMATE'.
(Other definitions for inmate that I've seen before include "Cell resident" , "One is confined to an institution" , "One living in (prison)" , "Convict" , "One is in jail" .)