Holding power opening very big meeting (10)
I believe the answer is:
possession
'holding' is the definition.
(possessing or holding something)
'power opening very big meeting' is the wordplay.
'opening' suggests taking the first letters.
'very big' becomes 'os' (abbreviation for outsize).
'meeting' becomes 'session' (session is a kind of meeting).
The initial letter of 'power' is 'p'.
'p'+'os'+'session'='POSSESSION'
(Other definitions for possession that I've seen before include "Haunting" , "Ownership" , "title" .)