Flyer soon welcomed by workers’ association (7)
I believe the answer is:
tinamou
'flyer' is the definition.
(tinamou is a kind of bird)
'soon welcomed by workers' association' is the wordplay.
'soon' becomes 'in a mo' (in a mo).
'welcomed by' indicates putting letters inside (letters welcomed inside).
'workers' association' becomes 'TU' (abbreviation for trade union).
'inamo' inserted inside 'tu' is 'TINAMOU'.
(Other definitions for tinamou that I've seen before include "US bird" , "Bird related to rhea" , "South American bird" , "Grouse-like tropical bird" , "S. American partridge-like bird" .)