Principal type of metal covering a support (8)
I believe the answer is:
maintain
'support' is the definition.
(I've seen this before)
'principal type of metal covering a' is the wordplay.
'principal' becomes 'main' (both can mean major).
'type of metal' becomes 'tin' ().
'covering' means one lot of letters goes inside another.
'tin' placed around 'a' is 'tain'.
'main'+'tain'='MAINTAIN'
(Other definitions for maintain that I've seen before include "Keep (at the same level)" , "Keep (a building) in good condition" , "Take the view or keep up repairs" , "Keep - support" , "Keep going" .)