Obtain old clothing if not waterproof material (4-3)

'waterproof material' is the definition.
(gore-tex is a waterproof material)

'obtain old clothing if not' is the wordplay.
'obtain' becomes 'get' (synonyms).
'old' becomes 'ex' (prefix meaning former or onetime).
'clothing' indicates putting letters inside (some letters cover up or clothe others).
'if not' becomes 'or' (both can be said before introducing an alternative).
'get'+'ex'='getex'
'getex' enclosing 'or' is 'GORE-TEX'.