Ultimately Mary Seacole was right (3)
I believe the answer is:
yes
'right' is the definition.
(thesaurus)
'ultimately mary seacole was' is the wordplay.
'ultimately' indicates one should take the final letters.
The last letters of 'mary seacole was' are 'YES'.
(Other definitions for yes that I've seen before include "3 across" , "That's 27 [RIGHT]" , "Oui" , "One response to "will you marry me?"" , "Word of agreement" .)