After he's used to show a monkey? (6)
I believe the answer is:
rhesus
'a monkey?' is the definition.
(rhesus is a kind of monkey)
'after he's used to show' is the wordplay.
'to show' says the answer is hidden in the clue.
'RHESUS' is hidden in the letters of 'after hes used'.
(Other definitions for rhesus that I've seen before include "Small, long-tailed monkey" , "Asian monkey - blood factor" , "Type of small monkey" , "(Kind of monkey) used in medical experiments" , "Kind of monkey (or factor)" .)