Demonstration by company for entertainment (4,8)
I believe the answer is:
show business
'entertainment' is the definition.
(I've seen this before)
'demonstration by company' is the wordplay.
'demonstration' becomes 'show' (show is a kind of demonstration).
'by' means one lot of letters go next to another.
'company' becomes 'business' (I've seen this before).
'show'+'business'='SHOW-BUSINESS'
'for' acts as a link.
(Other definitions for show business that I've seen before include "The world of entertainment" , "Entertainment industry" , "Theatre/music industry" .)