High point when Carmen's screened regularly, duck (9)
I believe the answer is:
crescendo
'high point' is the definition.
(I've seen this before)
'carmen's screened regularly duck' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'carmens'+'screened'='carmensscreened'
The alternate letters of 'carmensscreened' are 'crescend'.
'crescend'+'o'='CRESCENDO'
'when' is the link.
(Other definitions for crescendo that I've seen before include "Gradual increase in volume" , "increase in sound level" , "Conceders (anag.)" , "Gradual increase in music's loudness" , "Increase of volume in music" .)