Stare furtively - but keep back! (4)
I believe the answer is:
'stare furtively' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'keep back' is the wordplay.
'back' says the letters should be written backwards.
'keep' backwards is 'PEEK'.
'but' is the link.
Can you help me to learn more?
(Other definitions for peek that I've seen before include "Glimpse" , "Secret look" , "Take brief or surreptitious look" , "spy" , "Furtive glance" .)