Regularly valued bijou is sound (5)
I believe the answer is:
audio
'is sound' is the definition.
(I've seen this before)
'regularly valued bijou' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'valued'+'bijou'='valuedbijou'
The alternating letters of 'valuedbijou' are 'AUDIO'.
(Other definitions for audio that I've seen before include "Prefix relating to sound" , "Sound system" , "shown in reproduction" , "- - visual aids" , "Electronic sound reproduction" .)