In the past, impressionistic bird designs (8)
I believe the answer is:
patterns
'designs' is the definition.
(I know this)
'in the past impressionistic bird' is the wordplay.
'in the' indicates putting letters inside.
'past impressionistic' becomes 'pats' (I can't explain this - if you can you should give a lot more credence to this answer).
'bird' becomes 'tern' (tern is a kind of bird).
'pats' enclosing 'tern' is 'PATTERNS'.
Can you help me to learn more?
(Other definitions for patterns that I've seen before include "'Models, blueprints (8)'" , "Guides for making something" , "Designs, shapes" , "Repeated decorative designs" , "Models for making things" .)