Endless staying-power reflected in boat (3)
I believe the answer is:
tug
'boat' is the definition.
(tug is a kind of boat)
'endless staying-power reflected' is the wordplay.
'endless' means to remove the last letter.
'staying-power' becomes 'guts'.
'reflected' shows that the letters should be reversed in order.
'guts' with its last letter taken off is 'gut'.
'gut' back-to-front is 'TUG'.
'in' acts as a link.
(Other definitions for tug that I've seen before include "Pull hard; boat" , "Pull sharply" , "Boat to pull larger ones" , "Boat to pull larger one" , "Small ship for pulling" .)