Customs may see performance cutting costs (9)
I believe the answer is:
practices
'customs' is the definition.
(I've seen this before)
'performance cutting costs' is the wordplay.
'performance' becomes 'act' (act is a kind of performance).
'cutting' means one lot of letters goes inside another (some letters cut their way into a word).
'costs' becomes 'prices' ('price' can be a synonym of 'cost'**).
'act' placed inside 'prices' is 'PRACTICES'.
'may see' acts as a link.
(Other definitions for practices that I've seen before include "Customary ways of operation" , "Uses" , "Collections of doctors" , "Businesses of a doctor or lawyer" , "Activities, procedures or customs" .)