Fisherman's equipment picked up medium river creature (8)
I believe the answer is:
dormouse
'creature' is the definition.
(dormouse is a kind of animal)
'fisherman's equipment picked up medium river' is the wordplay.
'fisherman's equipment' becomes 'rod' (fishing rod).
'picked up' says the letters should be written backwards.
'medium' becomes 'm' (as in UK clothing sizes).
'river' becomes 'ouse' (Yorkshire river).
'rod' backwards is 'dor'.
'dor'+'m'+'ouse'='DORMOUSE'
(Other definitions for dormouse that I've seen before include "Small bushy-tailed rodent" , "Creature in teapot (Alice)" , "sleepy little creature" , "Alice tea party creature" , "Partygoer" .)