People with key arrive outside to open (8)
I believe the answer is:
commence
'open' is the definition.
(I've seen this before)
'people with key arrive outside' is the wordplay.
'people' becomes 'men' (man is a kind of person).
'with' means one lot of letters go next to another.
'key' becomes 'c' (I've seen this in other clues**).
'arrive' becomes 'come' (coming is a kind of arriving).
'outside' is an insertion indicator.
'men'+'c'='menc'
'menc' inserted within 'come' is 'COMMENCE'.
'to' acts as a link.
(Other definitions for commence that I've seen before include "Originate" , "Start, begin" , "Start up, set about" , "Kick off" , "open" .)