Act as a substitute for prop (5)
I believe the answer is:
stand
This clue is a triple definition!
'act' is the first definition.
(stand is a kind of act)
'as' acts as a link.
'a substitute for' is the second definition.
(thesaurus)
'prop' is the third definition.
(thesaurus)
(Other definitions for stand that I've seen before include "Tolerate; rise" , "Remain unaltered, tolerate" , "Platform; attitude" , "Set up" , "Remain stationary" .)