Keep an eye on reptile (7)
I believe the answer is:
monitor
I believe this clue is a double definition.
'keep an eye' is the first definition.
(I've seen this before)
'reptile' is the second definition.
(monitor is a kind of reptile)
'on' is the link.
(Other definitions for monitor that I've seen before include "A television or computer screen" , "Checking device - VDU - lizard" , "VDU or screen" , "Check on - large lizard" , "Supervise" .)