Principal means of support (4)
I believe the answer is:
arch
This clue might be a double definition.
'principal' is the first definition.
(as in arch-enemy)
'of support' is the second definition.
'arch' can be an answer for 'support'. I'm unsure of the 'of' bit.
'means' acts as a link.
Can you help me to learn more?
(Other definitions for arch that I've seen before include "Char the curved structure" , "Curved entrance" , "Instep of foot" , "Way though - on foot!" , "Cunning; principal" .)