Clambering with youngster (4)
I believe the answer is:
lamb
'youngster' is the definition.
(a lamb is a young sheep)
'clambering with' is the wordplay.
'with' indicates a hidden word.
'LAMB' is hidden within 'clambering'.
(Other definitions for lamb that I've seen before include "Youngster" , "Skipper" , "essayists?" , "author?" , "Meaty" .)