Mark held to account, ultimately (3)
I believe the answer is:
dot
'mark' is the definition.
(dotting is a kind of marking)
'held to account ultimately' is the wordplay.
'ultimately' indicates one should take the final letters.
The last letters of 'held to account' are 'DOT'.
(Other definitions for dot that I've seen before include "Absolutely punctual, on it" , "Point; speck" , "Spot, speck" , "Small round spot" , "Very small circular shape" .)