Old boy with very large settee, not oddly out-of-date (8)
I believe the answer is:
obsolete
'out-of-date' is the definition.
(synonyms)
'old boy with very large settee not oddly' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'with' says to put letters next to each other.
'very' becomes 'so' (both can mean 'extremely').
'large' becomes 'L' (eg in clothes sizes).
'not oddly' means one should take alternating letters (odd letters are removed).
The alternate letters of 'settee' are 'ete'.
'ob'+'so'+'l'+'ete'='OBSOLETE'
(Other definitions for obsolete that I've seen before include "being out of practice" , "Outmoded, passe" , "Obese lot may be out of date" , "'No longer used, out of date (8)'" , "Dead" .)