Victor's first to leave ring circling bull (5)
I believe the answer is:
inner
'ring circling bull' is the definition.
(ring circling the bullseye on a target)
'victor's first to leave' is the wordplay.
'victor' becomes 'winner' (synonyms).
'first to leave' means to remove the first letter (the first letter leaves).
'winner' with its initial letter removed is 'INNER'.
(Other definitions for inner that I've seen before include "Target zone" , "Esoteric" , "Deep" , "Central, internal" , "Inside; next to the bull (archery)" .)