Don’t forget when you are looking through the keyhole? (4,2,3,2)
I believe the answer is:
have an eye to
'don't forget' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.
'when you are looking through the keyhole?' is the wordplay.
I cannot really see how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is found within the answer.
'you' could be 'ye' (historical version of 'you') and 'ye' is found in the answer.
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?