Siam's capital has to pinch the post (5)
I believe the answer is:
stake
'post' is the definition.
(I know that post can be written as stake)
'siam's capital has to pinch the' is the wordplay.
'capital' indicates taking the first letters.
'has' becomes 'ake' (I can't justify this - if you can you should give a lot more credence to this answer).
'to pinch' indicates putting letters inside.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
The initial letter of 'siams' is 's'.
's'+'ake'='sake'
'sake' going around 't' is 'STAKE'.
Can you help me to learn more?
(Other definitions for stake that I've seen before include "Money placed as bet" , "Sum gambled" , "Amount wagered" , "Prize" , "Sharpened post; sum bet" .)