Make a mistake blocking novel exit for dog (3,7)

'dog' is the definition.
(fox terrier is a kind of dog)

'make a mistake blocking novel exit for' is the wordplay.
'make a mistake' becomes 'err' (to err is to make an error).
'blocking' is an insertion indicator (block can mean to plug a gap in something).
'novel' is an anagram indicator (letters in a new or novel order).
'exit'+'for'='exitfor'
'exitfor' anagrammed gives 'foxtier'.
'err' inserted inside 'foxtier' is 'FOX-TERRIER'.