Note politician yet to arrive, a pattern (8)
I believe the answer is:
template
'a pattern' is the definition.
(I've seen this before)
'note politician yet to arrive' is the wordplay.
'note' becomes 'te' (musical note).
'politician' becomes 'MP' (Member of Parliament).
'yet to arrive' becomes 'late'.
'te'+'mp'+'late'='TEMPLATE'
(Other definitions for template that I've seen before include "Pattern shape" , "original design" , "Design model" , "Pattern for document etc" , "Shaped pattern" .)