Curious old method of payment (3)
I believe the answer is:
odd
'curious' is the definition.
(curious can mean odd or peculiar)
'old method of payment' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'method of payment' becomes 'DD' (direct debit).
'o'+'dd'='ODD'
(Other definitions for odd that I've seen before include "Queer" , "Matchless" , "Abnormal" , "Casual" , "Rum" .)