Investigate means of payment, by the sound of it (5)
I believe the answer is:
check
'investigate' is the definition.
(I've seen this before)
'means of payment by the sound of it' is the wordplay.
'means of payment' becomes 'cheque' (I've seen this before).
'by the sound of it' shows a homophone (sound like).
'cheque' sounds like 'CHECK'.
(Other definitions for check that I've seen before include "Investigate; rein in" , "Restrain; square pattern" , "retard!" , "Investigate; stop" , "Verify or inspect as for accuracy" .)