While going back by river eat fish (7)
I believe the answer is:
sardine
'fish' is the definition.
(sardine is a type of fish)
'while going back by river eat' is the wordplay.
'while' becomes 'as' (eg 'while I walked' means 'as I walked').
'going back' is a reversal indicator.
'by' says to put letters next to each other.
'river' becomes 'r' (abbreviation).
'eat' becomes 'dine' (to dine is to eat a meal).
'as' backwards is 'sa'.
'sa'+'r'+'dine'='SARDINE'
(Other definitions for sardine that I've seen before include "Rinsed a (anag) - fish" , "Stonefish OR? Stone" , "Swimmer" , "(Cramped?) fish" , "Small pilchard" .)