Become aware of union cutting voucher (6)
I believe the answer is:
coupon
'voucher' is the definition.
('coupon' can be a synonym of 'voucher')
'become aware of union cutting' is the wordplay.
'become aware of' becomes 'copon' ('cop on' can informally mean 'become aware of').
'union' becomes 'u' (common abbreviation as in 'Trades Union Congress').
'cutting' indicates putting letters inside (some letters cut their way into a word).
'copon' enclosing 'u' is 'COUPON'.
(Other definitions for coupon that I've seen before include "Entry form" , "Voucher or ticket" , "betting slip" , "means to buy goods" , "Token" .)