Old saints, if meeting heresy, ultimately become rigid (6)
I believe the answer is:
ossify
'become rigid' is the definition.
(I've seen this before)
'old saints if meeting heresy ultimately' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'saints' becomes 'ss'.
'meeting' is a charade indicator (letters next to each other).
'ultimately' says to take the final letters.
The final letter of 'heresy' is 'y'.
'o'+'ss'+'if'+'y'='OSSIFY'
(Other definitions for ossify that I've seen before include "Become hard and bony with age" , "become inflexible" , "Change into bone, become rigid" , "become hardened" , "Get hard" .)