Advanced on Poles stopping union issuing flier (9)
I believe the answer is:
merganser
'issuing flier' is the definition.
'merganser' can be an answer for 'flier' (merganser is a kind of birdanimal). I'm not sure about the 'issuing' bit.
'advanced on poles stopping union' is the wordplay.
'advanced' becomes 'a' (abbreviation).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'poles' becomes 'NS' (north and south poles).
'stopping' means one lot of letters goes inside another (stopping can mean blocking a gap).
'union' becomes 'merger' (merger is a kind of union).
'a'+'ns'='ans'
'ans' inserted inside 'merger' is 'MERGANSER'.
Can you help me to learn more?
(Other definitions for merganser that I've seen before include "Sawbill" , "Diving bird" , "Diver" , "Diving duck" , "Together saw bill" .)