Firmly rooted in hollow that holds unusually big lake (12)
I believe the answer is:
incorrigible
'firmly rooted in hollow that holds unusually big lake' is the definition.
'incorrigible' can be an answer for 'firmly' (I've seen this before). I'm unsure of the rest of the definition.
'in hollow that holds unusually big lake' is the wordplay.
'hollow' becomes 'corrie' (geographical term).
'that holds' indicates putting letters inside.
'unusually' indicates anagramming the letters.
'lake' becomes 'L' (geographical abbreviation).
'big'+'l'='bigl'
'bigl' with letters rearranged gives 'gibl'.
'corrie' going around 'gibl' is 'corrigible'.
'in'+'corrigible'='INCORRIGIBLE'
Can you help me to learn more?
(Other definitions for incorrigible that I've seen before include "Firmly" , "Beyond correction" , "Beyond reform" , "Not reformable" , "beyond redemption" .)