Observe with a sly look back (4)
I believe the answer is:
keep
'observe' is the definition.
('keep' can be a synonym of 'observe')
'a sly look back' is the wordplay.
'a sly look' becomes 'peek' (I've seen this before).
'back' says the letters should be written in reverse.
'peek' written backwards gives 'KEEP'.
'with' acts as a link.
(Other definitions for keep that I've seen before include "Main tower of a medieval castle" , "Castle tower; cost of maintenance" , "Board and lodging" , "Conserve" , "honour" .)