Relentless pressure to stop redeployment of mail guy? (10)
I believe the answer is:
implacable
'relentless' is the definition.
(similar in meaning)
'pressure to stop redeployment of mail guy?' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'to stop' indicates putting letters inside (inserted letters plug or stop a gap).
'redeployment of' indicates anagramming the letters (letters deployed to new locations).
'guy?' becomes 'cable' (I've seen this in other clues).
'mail' is an anagram of 'imla'.
'p' going within 'imla' is 'impla'.
'impla'+'cable'='IMPLACABLE'
(Other definitions for implacable that I've seen before include "adamant" , "Unforgiving" , "such as can't be appeased" , "Pitiless, unyielding" , "Totally unrelenting" .)