Subject of certain theories, complicated yet trivial (10)

I believe the answer is:

relativity

'subject of certain theories' is the definition.
(as in Einstein's two theories of relativity)

'complicated yet trivial' is the wordplay.
'complicated' indicates anagramming the letters.
'yet'+'trivial'='yettrivial'
'yettrivial' with letters rearranged gives 'RELATIVITY'.

(Another definition for relativity that I've seen is " scientific principle".)

I've seen this clue in The Times.

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