Gazelle (8)
I believe the answer is:
antelope
'gazelle' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for antelope that I've seen before include "'Gazelle or springbok, say (8)'" , "it runs very quickly" , "A swift-running animal like gazelle" , "Lean poet (anag.)" , "horny beast!" .)