Bird, small, in midden, upside-down (6)
I believe the answer is:
peewit
'bird' is the definition.
(peewit is a kind of bird)
'small in midden upside-down' is the wordplay.
'small' becomes 'wee' (Scottish term for small).
'in' means one lot of letters goes inside another.
'midden' becomes 'tip' (I can't justify this - if you can you should give a lot more credence to this answer).
'upside down' is a reversal indicator.
'wee' placed into 'tip' is 'tiweep'.
'tiweep' written backwards gives 'PEEWIT'.
Can you help me to learn more?
(Other definitions for peewit that I've seen before include "Flighty type" , "Lapwing turned up in a wee pit" , "Lapwing - ie wept (anag)" , "Does it weep, this bird?" , "Kind of plover; let it weep" .)