Rough edge in cavity I have (9)
I believe the answer is:
primitive
'rough' is the definition.
(thesaurus)
'edge in cavity i have' is the wordplay.
'edge' becomes 'rim' (rim is a kind of edge).
'in' means one lot of letters goes inside another.
'cavity' becomes 'pit' ('pit' can be a synonym of 'cavity').
'i have' becomes 'ive'.
'rim' going within 'pit' is 'primit'.
'primit'+'ive'='PRIMITIVE'
(Other definitions for primitive that I've seen before include "At a very early stage of development" , "Old painter" , "Ancient" , "Original - uncivilised" , "Not sophisticated" .)