For fliers, a half pint is plenty (6)
I believe the answer is:
pilots
'for fliers' is the definition.
(pilot is a kind of flier)
'a half pint is plenty' is the wordplay.
'a half' indicates taking half.
'is plenty' becomes 'lots' (I've seen this before).
'pint' halved is 'pi'.
'pi'+'lots'='PILOTS'
(Other definitions for pilots that I've seen before include "They fly aircraft and guide ships into harbour" , "Aircraft operators" , "Shepherds" , "Airmen" , "They guide ships in and out of harbours" .)