Chief support for principal to remain (8)
I believe the answer is:
mainstay
'chief support' is the definition.
(I've seen this before)
'principal to remain' is the wordplay.
'principal' becomes 'main' (both can mean major).
'to remain' becomes 'stay' ('stay' can be a synonym of 'remain').
'main'+'stay'='MAINSTAY'
'for' acts as a link.
(Other definitions for mainstay that I've seen before include "Strongest support" , "Pillar of strength" , "big support" , "Rope to the mast is the chief support" , "Chief support, like mast cable" .)