Useful kind of cheque left in financial institution (5)
I believe the answer is:
blank
'useful kind of cheque' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'left in financial institution' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'financial institution' becomes 'bank' (bank is a kind of financial institution).
'l' inserted inside 'bank' is 'BLANK'.
Can you help me to learn more?
(Other definitions for blank that I've seen before include "Not written upon" , "Bare; not marked" , "Empty space" , "Vacuous" , "Not written on; (look) uncomprehending" .)