Musical effect always used in RB (6)
I believe the answer is:
reverb
'musical effect' is the definition.
(I've seen this before)
'always used in rb' is the wordplay.
'always' becomes 'ever' (synonyms).
'used in' is an insertion indicator.
'ever' going within 'rb' is 'REVERB'.
(Other definitions for reverb that I've seen before include "Effect where amplified sound vibrates" , "music made by this device" , "Sound prolongation" , "(Device for producing) an artificial musical acoustic" , "Echo in music" .)