Breeds pigs at main sty regularly to be immune (9)
I believe the answer is:
resistant
'be immune' is the definition.
(synonyms)
'breeds pigs at main sty regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'breeds'+'pigs'+'at'+'main'+'sty'='breedspigsatmainsty'
The alternate letters of 'breedspigsatmainsty' are 'RESISTANT'.
'to' acts as a link.
(Other definitions for resistant that I've seen before include "unaffected" , "Hindering the conductor" , "tough" , "Being hostile" , "Impervious (to corrosion, heat, etc)" .)