Youngster clambering around (4)
I believe the answer is:
lamb
'youngster' is the definition.
(a lamb is a young sheep)
'clambering around' is the wordplay.
'around' indicates the answer is hidden within the clue.
'LAMB' can be found hidden inside 'clambering'.
(Other definitions for lamb that I've seen before include "author?" , "Young sheep" , "Meaty" , "Charles ---, essayist; meek animal" , "A follower" .)