Tangled root in a lake (7)
I believe the answer is:
ontario
'lake' is the definition.
(I know that Ontario is a lake)
'tangled root in a' is the wordplay.
'tangled' is an anagram indicator.
'root'+'in'+'a'='rootina'
'rootina' with letters rearranged gives 'ONTARIO'.
(Other definitions for ontario that I've seen before include "Root in a Great Lake" , "Most populous Canadian province" , "city" , "part of N America" , "part of Canada" .)