Side with monarch and Scottish banker providing support (8)

'support' is the definition.
(rope supporting a ship's foremast)

'side with monarch and scottish banker' is the wordplay.
I cannot really see how this works, but
'side' could be 'head' (head is a kind of side) and 'head' is present in the answer.
'scottish banker' could be 'tay' (Tay is a Scottish river - something with banks) and 'tay' is found within the answer.
A single letter 's' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

'providing' acts as a link.