Purity of peas cultivated by relative (7)
I believe the answer is:
asepsis
'purity' is the definition.
(thesaurus)
'peas cultivated by relative' is the wordplay.
'cultivated' is an anagram indicator.
'by' says to put letters next to each other.
'relative' becomes 'sis' (short for sister).
'peas' is an anagram of 'asep'.
'asep'+'sis'='ASEPSIS'
'of' acts as a link.
(Other definitions for asepsis that I've seen before include "germ-free state" , "Absence of viruses and other microorganisms" , "Given freedom from infection" , "exclusion of micro-organisms" , "freedom from germs" .)