Whaler - a hard sailor (4)
I believe the answer is:
ahab
'whaler' is the definition.
(I have seen 'Fictional whaler ' mean 'ahab' so perhaps 'whaler' could also mean 'ahab')
'a hard sailor' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'sailor' becomes 'ab' (abbreviation for able seaman).
'a'+'h'+'ab'='AHAB'
(Other definitions for ahab that I've seen before include "Whaling Captain" , "Name of captain seeking Moby Dick" , "sea captain" , "Jezebel's husband, wicked king in the 'Old testament'" , "Israelite king" .)