Old coin adult found in passage (5)
I believe the answer is:
ducat
'old coin' is the definition.
(I've seen this before)
'adult found in passage' is the wordplay.
'adult' becomes 'a'.
'found in' indicates putting letters inside.
'passage' becomes 'duct' (duct is a kind of passage**).
'a' going into 'duct' is 'DUCAT'.
(Other definitions for ducat that I've seen before include "Old gold coin in e.g. Venice" , "tender once" , "Coin once used" , "Money no longer" , "Formerly a gold coin of several European countries" .)