Unkempt youngster maintains it's meaningless? (5)
I believe the answer is:
empty
'meaningless?' is the definition.
(similar in meaning)
'unkempt youngster maintains' is the wordplay.
'maintains' indicates a hidden word (I've seen 'maintaining' mean this (the solution is held or maintained within the clue)).
'EMPTY' is hidden in the letters of 'unkempt youngster'.
'it's' is the link.
(Other definitions for empty that I've seen before include "Evacuate" , "left for the milkman?" , "Hollow or vacant" , "Containing nothing" , "Cleared out" .)