Father’s taking motorway in vast plain (6)
I believe the answer is:
pampas
'vast plain' is the definition.
(the Pampas are South American plains)
'father's taking motorway' is the wordplay.
'father's' becomes 'papas' (papa is a kind of father).
'taking' is an insertion indicator.
'motorway' becomes 'M' (as in motorway names - Mp, Ma etc).
'papas' placed around 'm' is 'PAMPAS'.
'in' is the link.
(Other definitions for pampas that I've seen before include "Plains with grass" , "Treeless plain(s)" , "Vast plains of South America" , "South American plain" , "Prairies (with grass)" .)