Principal has to remain as chief support (8)
I believe the answer is:
mainstay
'chief support' is the definition.
(I've seen this before)
'principal has to remain' is the wordplay.
'principal' becomes 'main' (both can mean major).
'has' is a charade indicator (letters next to each other).
'to remain' becomes 'stay' (synonyms).
'main'+'stay'='MAINSTAY'
'as' acts as a link.
(Other definitions for mainstay that I've seen before include "chief supporter" , "big support" , "Strongest support" , "Most important support" , "Linchpin, principal support" .)