Having captured Troy, oracle castrated bull (5)
I believe the answer is:
steer
'castrated bull' is the definition.
(steer can mean a castrated bull)
'having captured troy oracle' is the wordplay.
'having captured' means one lot of letters goes inside another.
'troy' becomes 't'.
'oracle' becomes 'seer' (I've seen this before).
't' going into 'seer' is 'STEER'.
(Other definitions for steer that I've seen before include "Guide, direct" , "Handle" , "Direct; young ox" , "Guide - castrated ox" , "Manoeuvre" .)