Reptile, exhausted, going round English river (7)
I believe the answer is:
serpent
'reptile' is the definition.
(I know that serpent is a type of reptile)
'exhausted going round english river' is the wordplay.
'exhausted' becomes 'spent' (similar in meaning).
'going round' indicates putting letters inside (some letters go around others).
'english' becomes 'E' (abbreviation).
'river' becomes 'r' (abbreviation).
'e'+'r'='er'
'spent' placed around 'er' is 'SERPENT'.
(Other definitions for serpent that I've seen before include "Early wind instrument" , "Instrument of the past" , "Scaly reptile" , "Tempting" , "Present for a snake" .)