River always going in opposite directions (6)
I believe the answer is:
severn
'river' is the definition.
(River Severn)
'always going in opposite directions' is the wordplay.
'always' becomes 'ever' (synonyms).
'going in' indicates putting letters inside.
'opposite' says the letters should be written in reverse (I've seen 'opposite of' mean this).
'directions' becomes 'ns' (N is an example).
'ns' written backwards gives 'sn'.
'ever' placed into 'sn' is 'SEVERN'.
(Other definitions for severn that I've seen before include "Shrewsbury's river" , "A major British river" , "Britain's longest river" , "Welsh and English river" , "River with famous bore" .)