Having crossed valley, Maureen and Edward sat (8)

'sat' is the definition.
(model can mean to sit for a portrait or photograph)

'having crossed valley maureen and edward' is the wordplay.
'having crossed' means one lot of letters goes inside another.
'valley' becomes 'dell' (dell is a kind of valley).
'maureen' becomes 'Mo' (short for Maureen).
'and' is a charade indicator (letters next to each other).
'edward' becomes 'Ed' (short for Edward).
'mo'+'ed'='moed'
'dell' put into 'moed' is 'MODELLED'.