Prop up a broken-down horse (5)
I believe the answer is:
shore
'prop up' is the definition.
(I know that prop up can be written as shore)
'a broken-down horse' is the wordplay.
'a broken down' indicates an anagram.
'horse' with letters rearranged gives 'SHORE'.
(Other definitions for shore that I've seen before include "Stay in" , "Sailors might, given such leave" , "coastal area" , "Land by sea's edge" , "support being only temporary?" .)