Bangladesh's opener with sick note (4)
I believe the answer is:
bill
'note' is the definition.
(I know that note can be written as bill)
'bangladesh's opener with sick' is the wordplay.
'opener' indicates taking the first letters.
'with' means one lot of letters go next to another.
'sick' becomes 'ill' (similar in meaning).
The first letter of 'bangladesh' is 'b'.
'b'+'ill'='BILL'
(Other definitions for bill that I've seen before include "Law proposal" , "announce" , "Cosby or Clinton?" , "Parliamentary draft of proposed law" , "Informal first name for ex-president Clinton" .)