Snake crosses border by one old province (7)
I believe the answer is:
bohemia
'old province' is the definition.
'bohemia' can be an answer for 'province' (I have seen 'province of Europe' mean 'bohemia' so perhaps 'province' could also mean 'bohemia'). I am not sure about the 'old' bit.
'snake crosses border by one' is the wordplay.
'snake' becomes 'boa' (boa is a kind of snake).
'crosses' indicates putting letters inside.
'border by' becomes 'hem' (I've seen this before. I am not sure about the 'by' bit.).
'one' becomes 'I' (Roman numeral).
'hem'+'i'='hemi'
'boa' enclosing 'hemi' is 'BOHEMIA'.
Can you help me to learn more?
(Other definitions for bohemia that I've seen before include "Part of the Czech Republic" , "old kingdom" , "District for unconventional people - former European kingdom" , "Czechs seen here" , "Region of central Europe" .)