Dog otter struggling with large weir (10)
I believe the answer is:
rottweiler
'dog' is the definition.
(Rottweiler is an example)
'otter struggling with large weir' is the wordplay.
'struggling with' indicates anagramming the letters (the letters struggle or wriggle into a new form).
'large' becomes 'L' (eg in clothes sizes).
'l'+'weir'='lweir'
'otter' with its letters anagrammed with 'lweir' can make 'ROTTWEILER'.
(Other definitions for rottweiler that I've seen before include "Large black/tan dog" , "Highly aggressive sort" , "Fierce dog" , "Dog used as guard - let wire rot (anag)" , "Powerful German dog" .)