Slightly miffed at first by ruined idyll (6)
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" .)