River always confined by poles (6)
I believe the answer is:
severn
'river' is the definition.
(River Severn)
'always confined by poles' is the wordplay.
'always' becomes 'ever' (synonyms).
'confined by' is an insertion indicator.
'poles' becomes 'sn' (North and South poles).
'ever' going inside 'sn' is 'SEVERN'.
(Other definitions for severn that I've seen before include "River of England and Wales" , "Gloucester's river" , "Welsh and English river" , "A major British river" , "West Country river" .)