Watch carefully (3)
'watch carefully' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for observe that I've seen before include "Notice, watch" , "What this newspaper does?" , "Witness, watch" , "Discern" , "Pay attention" .)
(Other definitions for monitor that I've seen before include "Watch - lizard - warship - VDU" , "one checks continuously" , "Large lizard; TV screen" , "Keep an eye on" , "Check on - large lizard" .)
'watch carefully' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for eye that I've seen before include "See 1" , "Light-detector" , "May be private" , "where the pupil is seated" , "Observer" .)