See a square dog (6)
I believe the answer is:
canine
'dog' is the definition.
(I've seen this before)
'see a square' is the wordplay.
'see' becomes 'C' (the word for the letter, according to Chambers).
'square' becomes 'nine' (nine is a square number).
'c'+'a'+'nine'='CANINE'
(Other definitions for canine that I've seen before include "Of or like dogs" , "Tooth; dog" , "Relating to dogs" , "such as Lassie" , "Boxer" .)