Doubter worried and finally butted in (8)
I believe the answer is:
obtruded
'butted in' is the definition.
(I've seen this before)
'doubter worried and finally' is the wordplay.
'worried' indicates an anagram.
'finally' suggests the final letters.
The last letter of 'and' is 'd'.
'doubter' with letters rearranged gives 'obtrude'.
'obtrude'+'d'='OBTRUDED'
(Other definitions for obtruded that I've seen before include "Thrust oneself in" , "Came to prominence in an unwelcome way" , "Butted in" .)