Delay mail, accepting US pressure (7)
I believe the answer is:
suspend
'delay' is the definition.
(I know that defer is the is a more specific form of the action of suspend.)
'mail accepting us pressure' is the wordplay.
'mail' becomes 'send' (synonyms).
'accepting' indicates putting letters inside (inserted letters are accepted).
'pressure' becomes 'p' (scientific abbreviation).
'us'+'p'='usp'
'send' enclosing 'usp' is 'SUSPEND'.
(Other definitions for suspend that I've seen before include "Carry out hanging" , "Temporarily render inoperative" , "Keep in abeyance" , "Adjourn" , "Sends up (anag)" .)