His is not a job for a boy (7)
I believe the answer is:
postman
'his is not a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'job for a boy' is the wordplay.
'job' becomes 'post' (post is a kind of job).
'for' says to put letters next to each other.
'a boy' becomes 'man' ().
'post'+'man'='POSTMAN'
Can you help me to learn more?
(Other definitions for postman that I've seen before include "Letter deliverer" , "No stamp needed, stangely, for this job" , "one delivers" , "Chap with letters" , "TV's Pat, for instance" .)