Youngster clambering about (4)
I believe the answer is:
lamb
'youngster' is the definition.
(a lamb is a young sheep)
'clambering about' is the wordplay.
'about' indicates a hidden word.
'LAMB' can be found hidden inside 'clambering'.
(Other definitions for lamb that I've seen before include "Young sheep" , "Tender sheep meat" , "Young creature" , "Young shee" , "Young ovine" .)