River was rising around eastern kelp (7)
I believe the answer is:
seaweed
'kelp' is the definition.
(I've seen this in another clue)
'river was rising around eastern' is the wordplay.
'river' becomes 'dee' (River Dee).
'rising' is a reversal indicator (going up in a down clue).
'around' is an insertion indicator.
'eastern' becomes 'e'.
'dee'+'was'='deewas'
'deewas' written backwards gives 'saweed'.
'saweed' placed around 'e' is 'SEAWEED'.
(Other definitions for seaweed that I've seen before include "Iodine source" , "Plant such as kelp" , "Principal ingredient of laverbread" , "Marine algae" , "what's been washed up" .)