Writes back first from a boarding house (7)
I believe the answer is:
pension
'a boarding house' is the definition.
(I've seen this before)
'writes back first' is the wordplay.
'writes' becomes 'pens' ('pen' can be a synonym of 'write').
'back' is a reversal indicator.
'first' becomes 'noi' (resembles No. 1).
'noi' written backwards gives 'ion'.
'pens'+'ion'='PENSION'
'from' is the link.
(Other definitions for pension that I've seen before include "residence abroad" , "Nice little hotel" , "Payment to traitor" , "Bed and breakfast or small hotel in France" , "Payment to retired persons a continental boarding house" .)