Are discerning what to do with cheque (4,3)
I believe the answer is:
make out
This clue is a double definition.
'are discerning' is the first definition.
(to make out is to discern or recognise)
'what to do with cheque' is the second definition.
(to make out a cheque is to write it)
(Other definitions for make out that I've seen before include "Distinguish; pretend" , "understand" , "Discern" , "Manage to see" , "Descry - succeed" .)