Small model accepts upper-class partner (6)
I believe the answer is:
spouse
'partner' is the definition.
(I know that partner can be written as spouse)
'small model accepts upper-class' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'model' becomes 'pose' (model can mean to pose).
'accepts' means one lot of letters goes inside another.
'upper-class' becomes 'u' (similar in meaning).
'pose' enclosing 'u' is 'pouse'.
's'+'pouse'='SPOUSE'
(Other definitions for spouse that I've seen before include "Possibly wife" , "Other half" , "mate" , "Consort" , "Very hot and dry" .)