Might hold saint's splintered backbone (8)
I believe the answer is:
mainstay
'backbone' is the definition.
(thesaurus)
'might hold saint's splintered' is the wordplay.
'might' becomes 'may' (I've seen this before).
'hold' means one lot of letters goes inside another.
'splintered' indicates anagramming the letters.
'saint' anagrammed gives 'insta'.
'may' going around 'insta' is 'MAINSTAY'.
(Other definitions for mainstay that I've seen before include "Linchpin, principal support" , "Pillar of strength" , "Strong cable to secure mast, chief support" , "Chief support, like mast cable" , "Strong cable that secures a mast, chief support generally" .)