Big birds' nest they finally placed in Great Lake (5)
I believe the answer is:
eyrie
'big birds' nest' is the definition.
'eyrie' can be an answer for 'nest' (eyrie is a kind of nest). I am unsure of the 'big birds'' bit.
'they finally placed in great lake' is the wordplay.
'finally' suggests the final letters.
'placed in' is an insertion indicator (I've seen this in other clues).
'great lake' becomes 'erie' (I've seen this in another clue).
The last letter of 'they' is 'y'.
'y' placed into 'erie' is 'EYRIE'.
Can you help me to learn more?
(Other definitions for eyrie that I've seen before include "Eagle's lair" , "retreat for hunter" , "Lofty nest of a bird of prey" , "Nest of 19 Across" , "lofty house" .)