Letters received, making job secure (7)
I believe the answer is:
postbag
'letters received' is the definition.
(I've seen this before)
'job secure' is the wordplay.
'job' becomes 'post' (post is a kind of job).
'secure' becomes 'bag' (bag can mean to secure).
'post'+'bag'='POSTBAG'
'making' acts as a link.
(Other definitions for postbag that I've seen before include "carrier for Royal Mail" , "What one receives collectively" , "one's letters" , "All delivered" , "Letters received" .)