Pointed performance included in cost is cheating (5,8)
I believe the answer is:
sharp practice
'is cheating' is the definition.
(I've seen this before)
'pointed performance included in cost' is the wordplay.
'pointed' becomes 'sharp' ('sharp' can be similar in meaning to 'pointed').
'performance' becomes 'act' (act is a kind of performance).
'included in' indicates putting letters inside.
'cost' becomes 'price' (price is a kind of cost**).
'act' put inside 'price' is 'practice'.
'sharp'+'practice'='SHARP PRACTICE'
(Other definitions for sharp practice that I've seen before include "Dodgy dealings" , "Dishonesty" , "Ethically dubious machination" , "Unscrupulous dealing" , "cheating" .)