Lines found in stretch of river Severn (5)
I believe the answer is:
verse
'lines' is the definition.
(lines of poetry)
'stretch of river severn' is the wordplay.
'stretch of' indicates a hidden word (a stretch of letters from the clue).
'VERSE' is hidden within 'river severn'.
'found in' acts as a link.
(Other definitions for verse that I've seen before include "literary offering" , "Stanza; poetry" , "Part of a song or poem" , "Section of a poem" , "Poetry, a poem" .)