Ideal open air frolic in Holland (9)
I believe the answer is:
nonpareil
'ideal' is the definition.
(I know that ideal can be written as nonpareil)
'open air frolic in holland' is the wordplay.
'frolic' indicates anagramming the letters.
'in' is an insertion indicator.
'holland' becomes 'nl'.
'open'+'air'='openair'
'openair' anagrammed gives 'onparei'.
'onparei' placed within 'nl' is 'NONPAREIL'.
(Other definitions for nonpareil that I've seen before include "like none other" , "Unrivalled; old type size" , "Without equal, unrivalled" , "A variety of apple" , "Paragon" .)