Juvenile wildebeest boys discovered when climbing (5)
I believe the answer is:
young
'juvenile' is the definition.
(I've seen this before)
'wildebeest boys discovered when climbing' is the wordplay.
'wildebeest' becomes 'gnu' (synonyms).
'discovered' indicates the central letters.
'when climbing' is a reversal indicator.
The central letters of 'boys' are 'oy'.
'gnu'+'oy'='gnuoy'
'gnuoy' backwards is 'YOUNG'.
(Other definitions for young that I've seen before include "Born recently" , "At an early stage of development" , "Juvenile" , "Newly-developed" , "Fry" .)