Overcoat is designed for pair? No! (9)
I believe the answer is:
rainproof
'overcoat' is the definition.
(an overcoat is a rainproof item of clothing)
'designed for pair? no' is the wordplay.
'designed' indicates an anagram (designed with the letters).
'for'+'pair'+'no'='forpairno'
'forpairno' is an anagram of 'RAINPROOF'.
'is' acts as a link.
(Other definitions for rainproof that I've seen before include "Impervious" , "coat" , "Like protective cover" , "protected from storms?" , "top of home should be this!" .)