Develop a link with old clay (6)
I believe the answer is:
kaolin
'clay' is the definition.
(I've seen this before)
'develop a link with old' is the wordplay.
'develop' indicates anagramming the letters.
'with' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'a'+'link'='alink'
'alink' anagrammed gives 'kalin'.
'kalin' placed around 'o' is 'KAOLIN'.
(Other definitions for kaolin that I've seen before include "China clay" , "Nil oak in the ceramic clay" , "Soft white clay, used for making fine porcelain, say" .)