Peg to prosper endlessly (5)
I believe the answer is:
dowel
'peg' is the definition.
(a dowel is a wooden peg)
'prosper endlessly' is the wordplay.
'prosper' becomes 'dowell' (I've seen this in another clue).
'endlessly' means to remove the last letter.
'dowell' with its last letter taken off is 'DOWEL'.
'to' is the link.
(Other definitions for dowel that I've seen before include "Wooden peg" , "Headless peg used for joining components" , "Peg used for fastening two adjacent parts of a structure" , "Peg to hold two pieces together" , "Cylindrical wooden pin" .)