Film music of Keith Emerson? (5)
I believe the answer is:
theme
'film music' is the definition.
(a musical theme)
'of keith emerson?' is the wordplay.
'of' says the answer is hidden in the clue.
'THEME' can be found hidden inside 'keith emerson'.
(Other definitions for theme that I've seen before include "Motif - 4 across" , "issue" , "notedly subject to variation" , "Topic, leitmotif" , "Subject (of talk, text, etc)" .)