Chief supporter of creating saint in a month (8)
I believe the answer is:
mainstay
'chief supporter' is the definition.
(I've seen this before)
'creating saint in a month' is the wordplay.
'creating' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'a month' becomes 'may' (May is an example).
'saint' is an anagram of 'insta'.
'insta' put into 'may' is 'MAINSTAY'.
'of' is the link.
(Other definitions for mainstay that I've seen before include "Linchpin, principle support" , "chief supporter" , "Bulwark" , "Most important support" , "Strongest support" .)