A square hole in coat (4)
I believe the answer is:
four
'a square' is the definition.
(four is a square number)
'hole in coat' is the wordplay.
'hole' becomes 'o'.
'in' is an insertion indicator.
'coat' becomes 'fur' (fur coat is a kind of coat).
'o' going into 'fur' is 'FOUR'.
(Other definitions for four that I've seen before include "Cardinal" , "Set of bridge players?" , "Two squared" , "Square number" , "Number of players in a doubles game" .)