Majestic old lady taking on duke (5)
I believe the answer is:
grand
'majestic' is the definition.
(thesaurus)
'old lady taking on duke' is the wordplay.
'old lady' becomes 'gran' (I've seen this before).
'taking on' means one lot of letters go next to another.
'duke' becomes 'd'.
'gran'+'d'='GRAND'
(Other definitions for grand that I've seen before include "Marvellous" , "GBP1,000" , "sum of money" , "Magnificent, imposing" , "Stately - splendid" .)