Short note: check it (4)
I believe the answer is:
chit
'short note' is the definition.
(I've seen this before)
'check it' is the wordplay.
'check' becomes 'ch' (I've seen this in other clues).
'ch'+'it'='CHIT'
(Other definitions for chit that I've seen before include "Informal receipt" , "Voucher; anag. of 6" , "struck" , "Short note of money owed or received" , "Impudent girl" .)