Delay when mail has broken open (8)
I believe the answer is:
postpone
'delay' is the definition.
(I know this)
'mail has broken open' is the wordplay.
'mail' becomes 'post' (synonyms).
'has' means one lot of letters go next to another.
'broken' indicates anagramming the letters.
'open' anagrammed gives 'pone'.
'post'+'pone'='POSTPONE'
'when' acts as a link.
(Other definitions for postpone that I've seen before include "Put off till another time" , "'Put off, delay (8)'" , "'Hold over, defer (8)'" , "Put off, defer" , "Put off till later" .)