Mail service made impression on job (10,4)
I believe the answer is:
registered post
'mail' is the definition.
(I know that registered post is a type of mail)
'service made impression on job' is the wordplay.
I cannot quite understand how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found in the answer.
'job' could be 'post' (post is a kind of job) and 'post' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for registered post that I've seen before include "important letters? - with 28a" , "Type of mail" , "Specially secure delivery" , "sign for this?" , "Special form of delivery" .)