Pair perform, beset by spite and improper conduct (11)
I believe the answer is:
malpractice
'improper conduct' is the definition.
(I've seen this before)
'pair perform beset by spite' is the wordplay.
'pair' becomes 'pr' (abbreviation).
'perform' becomes 'act' (acting is a kind of performing).
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'spite' becomes 'malice' (synonyms).
'pr'+'act'='pract'
'pract' inserted within 'malice' is 'MALPRACTICE'.
'and' acts as a link.
(Other definitions for malpractice that I've seen before include "Professional misconduct" , "unprofessionalism" , "abuse" , "Improper conduct" , "Improper professional behaviour" .)