Leitmotiv used by Keith Emerson (5)

I believe the answer is:

theme

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'leitmotiv' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for theme that I've seen before include "Leitmotiv" , "A subject or repeated melody" , "Short melody" , "notedly subject to variation" , "Topic; melody" .)

I've seen this clue in The Times and The Sun.

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