Ass kept hidden in apprehension, age-related (6)
I believe the answer is:
onager
'ass' is the definition.
(type of wild ass)
'hidden in apprehension age-related' is the wordplay.
'hidden in' indicates a hidden word.
'ONAGER' is hidden within 'apprehension age related'.
'kept' is the link.
(Other definitions for onager that I've seen before include "Asian wild ass - orange (anag)" , "Variety of wild ass found in central Asia" , "ancient weapon" , "Asiatic wild ass" , "Strangely, the wild ass might be orange" .)