Duck, swans, tercel mostly flap about – everyone knows it (4,6)
I believe the answer is:
open secret
'everyone knows it' is the definition.
(I've seen this before)
'duck swans tercel mostly flap' is the wordplay.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'swans' becomes 'pens' (pen is a kind of swan).
'mostly' means to remove the last letter.
'flap' indicates anagramming the letters (I've seen 'flapping' mean this (flap can mean to move or fly around)).
'tercel' with its last letter taken away is 'terce'.
'terce' with letters rearranged gives 'ecret'.
'o'+'pens'+'ecret'='OPEN SECRET'
'about' acts as a link.
(Other definitions for open secret that I've seen before include "common knowledge" , "Centre pose (anag.)" , "Everyone knows it" , "Generally known matter" , "Private matter actually known to many" .)