Go off at a tangent about son in rented accommodation (7)
I believe the answer is:
digress
'go off at a tangent' is the definition.
(to go off at a tangent is to digress from a topic)
'about son in rented accommodation' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'son' becomes 's' (genealogical abbreviation for son).
'in' means one lot of letters goes inside another.
'rented accommodation' becomes 'digs' (as in student digs).
're'+'s'='res'
'res' going into 'digs' is 'DIGRESS'.
(Other definitions for digress that I've seen before include "Wander from a topic" , "Ramble" , "Leave the main subject" , "Be sidetracked" , "Go off the point" .)