One, two, three and four back in house (5)
I believe the answer is:
villa
'house' is the definition.
(I know that villa is a type of house)
'one two three and four back' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'two three' becomes 'll' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'and' means one lot of letters go next to another.
'four' becomes 'IV' (Roman numerals for four).
'back' says the letters should be written backwards.
'a'+'ll'+'iv'='alliv'
'alliv' in reverse letter order is 'VILLA'.
'in' is the link.
Can you help me to learn more?
(Other definitions for villa that I've seen before include "Large country house" , "Type of house" , "SECOND PART OF 8" , "Aston ..., English Premier League club" , "See 3" .)