One can be taken in with room to spare (6)
I believe the answer is:
lodger
'spare' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'one can be taken in with room' is the wordplay.
I cannot really understand how this works, but
'one' could be 'er' (I've seen this in other clues) and 'er' is found in the answer.
This explanation may well be incorrect...
'to' is the link.
(Other definitions for lodger that I've seen before include "One renting a room in a house" , "One who lives in a hired room" , "One renting and living with the owner" , "Spare room renter" , "He pays for room and board" .)