Closest human perhaps circling bull? (9)
I believe the answer is:
proximate
'closest' is the definition.
(I've seen this before)
'human perhaps circling bull?' is the wordplay.
'human perhaps' becomes 'primate' (human is a kind of primate).
'circling' indicates putting letters inside (circling can mean going around or containing).
'bull?' becomes 'ox'.
'primate' enclosing 'ox' is 'PROXIMATE'.
(Other definitions for proximate that I've seen before include "Closest" , "Very close" , "immediately following" , "next door" .)