Coat covering old pair of braces (4)
I believe the answer is:
four
'pair of braces' is the definition.
(brace can mean two)
'coat covering old' is the wordplay.
'coat' becomes 'fur coat' (fur is a kind of coat).
'covering' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'fur' enclosing 'o' is 'FOUR'.
(Other definitions for four that I've seen before include "Number in quartet" , "Figure" , "Square root of sixteen" , "An even number" , "Set of bridge players?" .)