Be a high street stationer — it's a stable job (9)
I believe the answer is:
liveryman
'a stable job' is the definition.
(job in a stable)
'be a high street stationer' is the wordplay.
'be' becomes 'live' (to be is to live or exist).
'a high street stationer' becomes 'Ryman' (Ryman is a stationery company).
'live'+'ryman'='LIVERYMAN'
'it's' is the link.
(Other definitions for liveryman that I've seen before include "One looking after hacks" , "Member of guild" , "Uniformed servant" , "Member of a City Company" .)