Clue from winner eclipsing Boatman's last (4)
I believe the answer is:
hint
'clue from winner' is the definition.
'hint' can be an answer for 'clue' ('hint' can be a synonym of 'clue'). I'm not certain of the 'from winner' bit.
'eclipsing boatman's last' is the wordplay.
'eclipsing boatman's' becomes 'hin' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'last' becomes 't' (I've seen this in other clues).
'hin'+'t'='HINT'
Can you help me to learn more?
(Other definitions for hint that I've seen before include "Indirect clue" , "Clue - soupcon" , "Helpful suggestion" , "Indicate indirectly" , "Imply, suggest" .)