Company abandoned voucher: it's over! (4)
I believe the answer is:
upon
'over' is the definition.
(thesaurus)
'company abandoned voucher' is the wordplay.
'company' becomes 'co' (abbreviation).
'abandoned' is a deletion indicator.
'voucher' becomes 'coupon' (synonyms).
'coupon' with 'co' taken away is 'UPON'.
'it's' is the link.
(Other definitions for upon that I've seen before include "On tope of" , "Straddling" , "Carried - on top" , "Au courant" , "voucher, business stopped" .)