Pole is endlessly flash (4)
I believe the answer is:
spar
'pole' is the definition.
(I know that spar is a type of pole)
'endlessly flash' is the wordplay.
'endlessly' means to remove the last letter.
'flash' becomes 'spark' (spark is a kind of flash).
'spark' with its last letter removed is 'SPAR'.
'is' acts as a link.
(Other definitions for spar that I've seen before include "Do up (bird for oven)" , "Shadow-box" , "Scrap" , "Round pole to support sail" , "Pole-shaped part of the rigging" .)