Yes, one filling in was annoying (7)
I believe the answer is:
granted
'annoying' is the definition.
Both the answer and definition are adjectives. Maybe there's an association between them I don't understand?
'yes one filling in' is the wordplay.
I cannot quite see how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'was' acts as a link.
Can you help me to learn more?
(Other definitions for granted that I've seen before include "Admittedly; given" , "Admittedly; gave formally" , "Conceded" , "One doesn't like being taken for it" , "Given; allowed" .)