Keen -- before rehearsal shows cheating (5,8)
I believe the answer is:
sharp practice
'cheating' is the definition.
(I've seen this before)
'keen before rehearsal' is the wordplay.
'keen' becomes 'sharp' ('sharp' can be a synonym of 'keen'**).
'before' means one lot of letters go next to another.
'rehearsal' becomes 'practice' (I've seen this before).
'sharp'+'practice'='SHARP PRACTICE'
'shows' is the link.
(Other definitions for sharp practice that I've seen before include "Unscrupulous dealing" , "cheating" , "Dodgy dealings" , "Dishonesty" , "fiddle" .)