Support youngster endlessly (3)
I believe the answer is:
tee
'support' is the definition.
(tee is a kind of support)
'youngster endlessly' is the wordplay.
'youngster' becomes 'teen' (I've seen this before).
'endlessly' means to remove the last letter.
'teen' with its final letter removed is 'TEE'.
(Other definitions for tee that I've seen before include "Peg for golfer" , "Where a hole starts" , "Start of a hole in golf" , "Golfing support" , "Golf-peg" .)