Creature in river heartless oarsman grabs (3,4)
I believe the answer is:
roe deer
'creature' is the definition.
(I've seen this before)
'river heartless oarsman grabs' is the wordplay.
'river' becomes 'dee' (River Dee).
'heartless' means to remove the middle letters.
'oarsman' becomes 'rower' (synonyms).
'grabs' indicates putting letters inside.
'rower' with its centre taken out is 'roer'.
'dee' put into 'roer' is 'ROE-DEER'.
'in' acts as a link.
(Other definitions for roe deer that I've seen before include "beast" , "Small (antlered) mammal" , "how beastly is this!" , "Brute" , "Small graceful woodland animal" .)