Huge tailless rodent follows one around river (8)
I believe the answer is:
enormous
'huge' is the definition.
(synonyms)
'tailless rodent follows one around river' is the wordplay.
'tailless' means to remove the last letter.
'rodent' becomes 'mouse' (mouse is a kind of rodent).
'follows' says to put letters next to each other.
'around' is a reversal indicator.
'river' becomes 'r'.
'one' backwards is 'eno'.
'mouse' with its final letter taken away is 'mous'.
'eno'+'r'='enor'
'mous' after 'enor' is 'ENORMOUS'.
(Other definitions for enormous that I've seen before include "Huge, really big" , "Huge, tremendous" , "'Huge, gigantic (8)'" , "Massive" , "Immense" .)