Sea snail is more fresh by river (5)
I believe the answer is:
ormer
'sea snail' is the definition.
(type of sea snail)
'more fresh by river' is the wordplay.
'fresh' indicates an anagram.
'by' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation for river).
'more' with letters rearranged gives 'orme'.
'orme'+'r'='ORMER'
'is' acts as a link.
(Other definitions for ormer that I've seen before include "Ear-shell" , "seashell" , "Seafood" , "Mollusc" , "sea creature" .)