Clambering around youngster (4)
I believe the answer is:
lamb
'youngster' is the definition.
(a lamb is a young sheep)
'clambering around' is the wordplay.
'around' indicates a hidden word.
'LAMB' can be found hidden inside 'clambering'.
(Other definitions for lamb that I've seen before include "author?" , "Noted essayist - symbol of innocence" , "Tender sheep meat" , "Youngster" , "essayists?" .)