Observing party policy, person's wise to keep schtoom in the end (2-7)

'observing party policy' is the definition.
(on-message means conforming to a political message)

'person's wise to keep schtoom in the end' is the wordplay.
'person's' becomes 'ones' (I've seen this in other clues).
'wise' becomes 'sage' (similar in meaning).
'to keep' means one lot of letters goes inside another.
'in the end' indicates one should take the final letters.
The final letter of 'schtoom' is 'm'.
'ones'+'sage'='onessage'
'onessage' going around 'm' is 'ON-MESSAGE'.