Snake Pass contains one (4)
I believe the answer is:
coil
'snake' is the definition.
(thesaurus)
'pass contains one' is the wordplay.
'pass' becomes 'col' (col is a kind of pass).
'contains' is an insertion indicator.
'one' becomes 'i' (Roman numeral**).
'col' enclosing 'i' is 'COIL'.
(Other definitions for coil that I've seen before include "Wind in a series of loops" , "Wind into rings or spiral" , "Curl, spiral" , "Series of concentric circles" , "Wire winding" .)