Dreadful, like a doughnut filled with jam, ultimately (8)
I believe the answer is:
alarming
'dreadful' is the definition.
(similar in meaning)
'like a doughnut filled with jam ultimately' is the wordplay.
'like' becomes 'a la' ('a la' can mean 'in the way of' in French).
'a doughnut' becomes 'ring' (synonyms).
'filled with' is an insertion indicator.
'ultimately' says to take the final letters.
The final letter of 'jam' is 'm'.
'ring' going around 'm' is 'rming'.
'ala'+'rming'='ALARMING'
(Other definitions for alarming that I've seen before include "Marginal (anag)" , "Striking with fear" , "Worrying or disturbing" , "Scary, frightening" , "Disturbing, frightening" .)