Dog runs into lake in extremes of terror (7)
I believe the answer is:
terrier
'dog' is the definition.
(I know that terrier is a type of hunting dog)
'runs into lake in extremes of terror' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'into' is an insertion indicator.
'lake' becomes 'erie' (Lake Erie is one of the Canadian/US 'Great Lakes').
'in' means one lot of letters goes inside another.
'extremes of' means to remove the middle letters (only the letters at the extremes of the word).
'terror' with its middle taken out is 'tr'.
'r' placed inside 'erie' is 'errie'.
'errie' put into 'tr' is 'TERRIER'.
(Other definitions for terrier that I've seen before include "he's not a boxer" , "Breed of dog - Territorial Army volunteer" , "See 11" , "Small hunting dog" , "Persistent person" .)