Observe when you look back (4)
I believe the answer is:
keep
'observe' is the definition.
(I know that observe can be written as keep)
'look back' is the wordplay.
'look' becomes 'peek' (peek can mean to look at quickly).
'back' shows that the letters should be reversed in order.
'peek' back-to-front is 'KEEP'.
'when you' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for keep that I've seen before include "Board and lodgings, perhaps" , "honour" , "Continue" , "Retain, maintain" , "Living" .)