Musical idea Keith Emerson takes in (5)
I believe the answer is:
theme
'musical idea' is the definition.
(as in a musical motif or theme)
'keith emerson takes in' is the wordplay.
'in' indicates the answer is hidden within the clue.
'THEME' can be found hidden inside 'keith emerson takes'.
(Other definitions for theme that I've seen before include "Subject (on which person writes, speaks)" , "Unifying motif" , "essay" , "Subject set for discussion" , "Motif that's repeated" .)