In old Thailand, Sam gets out of line (4)
I believe the answer is:
siam
'in old thailand' is the definition.
'Siam' can be an answer for 'thailand' (synonyms). I'm unsure of the 'in old' bit.
'sam gets out of line' is the wordplay.
'gets' means one lot of letters goes inside another (gets can mean captures or absorbs).
'out of line' becomes 'i' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sam' going around 'i' is 'SIAM'.
Can you help me to learn more?
(Other definitions for siam that I've seen before include "Old Thailand" , "Thailand, formerly" , "The King and I nation" , "Thailand's former name" , "old-style country" .)