His accounts may be all at sea (6)
I believe the answer is:
purser
'his accounts may be all at sea' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for purser that I've seen before include "One contracting to be" , "Ship's accountant" , "Ship's officer attending to the welfare of passengers" , "Ship's steward" , "Ship's officer responsible for the accounts etc" .)