He's after big things at sea (6)
I believe the answer is:
whaler
'he's after big things at sea' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he's' is the wordplay.
I cannot quite understand how this works, but
'he' could be 'hale' and 'hale' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for whaler that I've seen before include "one may go to work at sea" , "Aussie tramp?" , "boat carrying harpoons" , "Mammal-hunting boat" , "Seaman like Ahab" .)