After cloudiness at first, Fair Isle otherwise clears up (9)

'clears up' is the definition.
('clarify' can be a synonym of 'clear up')

'after cloudiness at first fair isle otherwise' is the wordplay.
'after' means one lot of letters go next to another.
'at first' says to take the initial letters.
'otherwise' is an anagram indicator.
The initial letter of 'cloudiness' is 'c'.
'fair'+'isle'='fairisle'
'fairisle' with letters rearranged gives 'larifies'.
'c'+'larifies' is 'CLARIFIES'.