One can board if there's room to spare (6)
I believe the answer is:
lodger
'one' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'board if there's room to spare' is the wordplay.
I cannot quite see how this works, but
'board' could be 'lodge' (both can mean to stay in an establishment) and 'lodge' is found within the answer.
A single letter 'r' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'can' is the link.
Can you help me to learn more?
(Other definitions for lodger that I've seen before include "Spare room renter" , "One renting and living with the owner" , "Long-term paying guest" , "One paying for accommodation" , "Renter who lives with the owner" .)