Raptor starts to eat allosaurus, gauging left eyeball (5)
I believe the answer is:
eagle
'raptor' is the definition.
(eagle is a kind of raptor)
'starts to eat allosaurus gauging left eyeball' is the wordplay.
'starts to' suggests taking the first letters.
The initial letters of 'eat allosaurus gauging left eyeball' is 'EAGLE'.
(Other definitions for eagle that I've seen before include "high-flier" , "part of round that's exceptional" , "Bird with hooked bill and long broad wings" , "The bald one is a US symbol" , "Valuable piece" .)