Warning poster having proposal about Ulster? (10)
I believe the answer is:
admonition
'warning' is the definition.
('admonition' can be a synonym of 'warning')
'poster having proposal about ulster?' is the wordplay.
'poster' becomes 'ad'.
'having' is a charade indicator (letters next to each other).
'proposal' becomes 'motion' (motion is a kind of proposal**).
'about' indicates putting letters inside.
'ulster?' becomes 'ni' (Northern Ireland).
'motion' enclosing 'ni' is 'monition'.
'ad'+'monition'='ADMONITION'
(Other definitions for admonition that I've seen before include "Rebuke" , "reprimand" .)