Real ones stay in at night (5)
I believe the answer is:
teeth
'real ones stay in at night' is the definition.
I can't tell whether this definition defines the answer.
'real ones stay in' is the wordplay.
'real' becomes 'e' (e is a kind of real).
'ones stay' becomes 'teth' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'e' inserted within 'teth' is 'TEETH'.
Can you help me to learn more?
(Other definitions for teeth that I've seen before include "Genuine effectiveness" , "They may be given crowns" , "Bony structures for eating" , "cogs in the machine, perhaps" , "watchdog needs these" .)