River always between south and north (6)
I believe the answer is:
severn
'river' is the definition.
(River Severn)
'always between south and north' is the wordplay.
'always' becomes 'ever' (synonyms).
'between' indicates putting letters inside.
'south' becomes 'S' (abbreviation).
'and' says to put letters next to each other.
'north' becomes 'N' (abbreviation).
's'+'n'='sn'
'ever' going into 'sn' is 'SEVERN'.
(Other definitions for severn that I've seen before include "Major UK river" , "Worcester's river" , "Longest UK river" , "River flowing into the Bristol Channel" , "Britian's longest river" .)