Mike indeed guards against losing extremely accommodating principal support (8)
I believe the answer is:
mainstay
'principal support' is the definition.
(I've seen this before)
I cannot understand how the rest of the clue works.
(Other definitions for mainstay that I've seen before include "Chief source of support" , "Strong cable to secure mast, chief support" , "Most important support" , "Bulwark" , "Strongest support" .)