You may not remember having spent the night in one (3)
I believe the answer is:
cot
'night in one' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'you may not remember having spent the' is the wordplay.
I cannot really understand how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
A single letter 'c' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cot that I've seen before include "Baby's bed" , "Small child's bed" , "baby carrier?" , "Small bed with high sides" , "Small boat to hold a baby" .)