Blunder by scientist ultimately creating a monster (6)
I believe the answer is:
terror
'a monster' is the definition.
(thesaurus)
'blunder by scientist ultimately' is the wordplay.
'blunder' becomes 'error' (I've seen this in another clue).
'by' is a charade indicator (letters next to each other).
'ultimately' suggests the final letters.
The final letter of 'scientist' is 't'.
'error' after 't' is 'TERROR'.
'creating' acts as a link.
(Other definitions for terror that I've seen before include "Abject fear" , "Troublesome child" , "Troublesome kid" , "Organised political violence" , "Fear - now being warred on" .)