Crossword Genius

Slightly miffed at first by ruined idyll (6)

Ross

I believe the answer is:

mildly

'slightly' is the definition.
('mildly' can be a synonym of 'slightly')

'miffed at first by ruined idyll' is the wordplay.
'at first' says to take the initial letters.
'by' is a charade indicator (letters next to each other).
'ruined' indicates anagramming the letters.
The initial letter of 'miffed' is 'm'.
'idyll' anagrammed gives 'ildly'.
'm'+'ildly'='MILDLY'

(Other definitions for mildly that I've seen before include "gentleness" , "Gently or moderately" , "In a gentle way" .)

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