Juvenile year-old antelope lying about (5)
I believe the answer is:
young
'juvenile' is the definition.
(I've seen this in another clue)
'year-old antelope lying about' is the wordplay.
'year' becomes 'y'.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'antelope lying' becomes 'gnu' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'about' says the letters should be written in reverse.
'gnu' reversed gives 'ung'.
'y'+'o'+'ung'='YOUNG'
Can you help me to learn more?
(Other definitions for young that I've seen before include "Offspring; immature" , "Mormon pioneer" , "issue" , "litter?" , "Offspring (of animals etc)" .)