The one sent off now as a replacement (7)
I believe the answer is:
instead
'as a replacement' is the definition.
(I've seen this in another clue)
'the one sent off now' is the wordplay.
'the one' becomes 'i' (Roman numeral).
'off' indicates an anagram.
'now' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'sent' is an anagram of 'nste'.
'i'+'nste'+'ad'='INSTEAD'
(Other definitions for instead that I've seen before include "In place (of)" , "In lieu" , "Stained (anag)" , "for a change" , "Rather - detains (anag)" .)