Interest in post (5)
I believe the answer is:
stake
I believe this is a double definition.
'interest' is the first definition.
(synonyms)
'post' is the second definition.
(synonyms)
'in' acts as a link.
(Other definitions for stake that I've seen before include "Financial interest" , "Ante, bet" , "Money gambled" , "Post; money for bet" , "Deposit as a wager" .)