Examine what's said to be a bill of exchange (5)
I believe the answer is:
check
'examine' is the definition.
(check can mean to examine or inspect)
'said to be a bill of exchange' is the wordplay.
'said to be' shows a homophone (sound like).
'a bill of exchange' becomes 'cheque' (cheque is a kind of bill of exchange).
'cheque' is a homophone of 'CHECK'.
'what's' is the link.
(Other definitions for check that I've seen before include "Restrain; vet" , "Verify or inspect as for accuracy" , "Examination" , "Refrain or verify" , "Investigate; stop" .)