On rising picked up a robe (4)
I believe the answer is:
toga
'robe' is the definition.
(I have seen 'Roman robe ' mean 'toga' so perhaps 'robe' could also mean 'toga')
'on rising picked up a' is the wordplay.
'on' is a charade indicator (letters next to each other).
'rising' is a reversal indicator (going up in a down clue).
'picked up' becomes 'got'.
'got' in reverse letter order is 'tog'.
'tog'+'a' is 'TOGA'.
(Other definitions for toga that I've seen before include "Got a cloak for an old Roman" , "Roman tunic" , "Cloak of ancient Rome" , "Robe worn by Caesar" , "Got a robe for an old Roman" .)