Number observed outside party, perhaps (9)
I believe the answer is:
seventeen
'number' is the definition.
(I know that seventeen is a Leyland number)
'observed outside party perhaps' is the wordplay.
'observed' becomes 'seen' (synonyms).
'outside' is an insertion indicator.
'party perhaps' becomes 'event' (party is a kind of event).
'seen' going around 'event' is 'SEVENTEEN'.
(Other definitions for seventeen that I've seen before include "next clue" , "Cardinal number" , "Figure" , "Quite a few" , "XVII" .)