Back up firm in mail (6)
I believe the answer is:
second
'back up' is the definition.
(seconding is a kind of backing up)
'firm in mail' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'in' is an insertion indicator.
'mail' becomes 'send' (synonyms).
'co' placed within 'send' is 'SECOND'.
(Other definitions for second that I've seen before include "Behind" , "Eighty six thousand and four hundredth of a day" , "bit" , "honours degree" , "tick" .)