Invalid rich son is kind of pasty (7)
I believe the answer is:
cornish
'kind of pasty' is the definition.
'invalid rich son' is the wordplay.
'invalid' indicates an anagram.
'rich'+'son'='richson'
'richson' is an anagram of 'CORNISH'.
'is' acts as a link.
(Other definitions for cornish that I've seen before include "Rich son (anag)" , "West Country" , "A Celtic language" , "From a far SW county" .)