Call about princess on horse? (6)
I believe the answer is:
riding
'on horse?' is the definition.
(I've seen this before)
'call about princess' is the wordplay.
'call' becomes 'ring' (telephone call).
'about' indicates putting letters inside.
'princess' becomes 'di' (Princess Diana).
'ring' enclosing 'di' is 'RIDING'.
(Other definitions for riding that I've seen before include "Sitting on and controlling the movements of a horse" , "Establishment teaching horsemanship" , "On top" , "County division for a sporting activity" , "Going cycling, perhaps" .)