Supporting duke's cross (4)
I believe the answer is:
ford
'cross' is the definition.
(to ford a river is to cross it)
'supporting duke's' is the wordplay.
'supporting' becomes 'for' (being for something means supporting it).
'duke' becomes 'd' (abbreviation).
'for'+'d'='FORD'
(Other definitions for ford that I've seen before include "Harrison - - (film actor)" , "US President - river crossing" , "Car firm" , "Henry ...., first mass producer of cars" , "Wade across" .)