Onlooker not to sit in collapsing byre (9)
I believe the answer is:
bystander
'onlooker' is the definition.
(I've seen this before)
'not to sit in collapsing byre' is the wordplay.
'not to sit' becomes 'stand' ('stand' is the opposite to 'sit').
'in' is an insertion indicator.
'collapsing' indicates an anagram.
'byre' anagrammed gives 'byer'.
'stand' placed inside 'byer' is 'BYSTANDER'.
(Other definitions for bystander that I've seen before include "Onlooker (often innocent)" , "Watcher of occurrence" , "Bad sentry may be an onlooker" , "Non-participant spectator" , "Non-partcipating spectator" .)