You may have forgotten sleeping in it (3)
I believe the answer is:
cot
'you may' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'forgotten sleeping in it' is the wordplay.
'forgotten sleeping in' becomes 'co' (I can't explain this - if you can you should give a lot more credence to this answer).
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'co'+'t'='COT'
'have' is the link.
Can you help me to learn more?
(Other definitions for cot that I've seen before include "Folding bed - fingerstall" , "Small child's bed" , "Crib" , "Baby's bed" , "Infant's bed" .)