Bunch of flyers, doubly good in strong wind (6)
I believe the answer is:
gaggle
'bunch of flyers' is the definition.
(group of geese)
'doubly good in strong wind' is the wordplay.
'doubly' indicates the doubling of some letters.
'good' becomes 'g' (abbreviation).
'in' indicates putting letters inside.
'strong wind' becomes 'gale' (I've seen this before).
'g' doubled is 'gg'.
'gg' inserted within 'gale' is 'GAGGLE'.
(Other definitions for gaggle that I've seen before include "Gathering of wandering geese" , "Flock of geese on the ground" , "Colective noun for geese" , "Collective noun for geese" , "Assembly of geese" .)