Easy marks posing as high-flyers? (7,5)
I believe the answer is:
sitting ducks
'easy marks' is the definition.
('sitting duck' can be a synonym of 'easy mark')
'posing as high-flyers?' is the wordplay.
'posing' becomes 'sitting' (as in sitting for a portrait).
'as high flyers?' becomes 'ducks' (I can't explain this - if you can you should give a lot more credence to this answer).
'sitting'+'ducks'='SITTING DUCKS'
Can you help me to learn more?
(Other definitions for sitting ducks that I've seen before include "Easy targets" , "small things not to be missed?" .)