Entering sustained campaign, contest comparatively less flat (6)

I believe the answer is:

wavier

'comparatively less flat' is the definition.
(something wavy is not flat)

'entering sustained campaign contest' is the wordplay.
'entering' means one lot of letters goes inside another.
'sustained campaign' becomes 'war'.
'contest' becomes 'vie' (I've seen this before).
'war' enclosing 'vie' is 'WAVIER'.

(Another definition for wavier that I've seen is " Having more undulations".)

I've seen this clue in The Times.

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