Keep an eye on lizard (7)
I believe the answer is:
monitor
I believe this is a double definition.
'keep an eye' is the first definition.
(I've seen this before)
'lizard' is the second definition.
(I know that monitor is a type of lizard)
'on' is the link.
(Other definitions for monitor that I've seen before include "A television or computer screen" , "Observe over time" , "Keep a watching brief" , "Keep track of" , "giant lizard" .)