First beast of burden being loaded close to the centre (8)
I believe the answer is:
proximal
'close to the centre' is the definition.
(anatomical term)
'first beast of burden being loaded' is the wordplay.
'first' becomes 'primal' (primal can mean early or first).
'beast of burden' becomes 'ox' (an ox is a bull used for farm work).
'being loaded' is an insertion indicator.
'primal' enclosing 'ox' is 'PROXIMAL'.
(Other definitions for proximal that I've seen before include "At the near end" , "XWD SETTER" .)