Perfect, a musical instrument enthralling bishop so (8)
I believe the answer is:
absolute
'perfect' is the definition.
(synonyms)
'a musical instrument enthralling bishop so' is the wordplay.
'musical instrument' becomes 'lute' (lute is a kind of musical instrument).
'enthralling' indicates putting letters inside.
'bishop' becomes 'B' (chess abbreviation).
'a'+'lute'='alute'
'b'+'so'='bso'
'alute' enclosing 'bso' is 'ABSOLUTE'.
(Other definitions for absolute that I've seen before include "rank" , "Unconditional, downright," , "Complete - unlimited" , "Unrestricted, out-and-out" , "No strings attached" .)