Extremely wary crossing lake in case (8)
I believe the answer is:
leeriest
'extremely wary' is the definition.
(leery can mean cautious or wary)
'crossing lake in case' is the wordplay.
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'lake' becomes 'erie' (Lake Erie is one of the Canadian/US 'Great Lakes').
'in case' becomes 'lest' (I've seen this before).
'erie' put within 'lest' is 'LEERIEST'.
(Other definitions for leeriest that I've seen before include "Most guarded" , "Most careful" .)