Crossword Genius

Most exciting preliminary contest involving fantastic side (8)

Ross

I believe the answer is:

headiest

'most exciting' is the definition.
('heady' can be similar in meaning to 'exciting')

'preliminary contest involving fantastic side' is the wordplay.
'preliminary contest' becomes 'heat' (I've seen this before).
'involving' means one lot of letters goes inside another.
'fantastic' indicates anagramming the letters.
'side' is an anagram of 'dies'.
'heat' going around 'dies' is 'HEADIEST'.

I've seen this clue in The Times.
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