Something in the air, perhaps whistled not badly (11)
I believe the answer is:
thistledown
'something in the air' is the definition.
(on thistle seeds flying in the air)
'whistled not badly' is the wordplay.
'badly' indicates anagramming the letters (letters in the wrong order).
'whistled'+'not'='whistlednot'
'whistlednot' is an anagram of 'THISTLEDOWN'.
'perhaps' is the link.
(Other definitions for thistledown that I've seen before include "What helps some seeds fly" , "Light fluff of some seeds" , "Airborne seeds" , "A mass of feathery seeds" , "Wind-blown seeds and fluff" .)