Annual letter comes in good time (6)
I believe the answer is:
yearly
'annual' is the definition.
(I know that annual can be written as yearly)
'letter comes in good time' is the wordplay.
I cannot quite see how this works, but
'letter' could be 'y' (Y is a letter of the alphabet) and 'y' is located in the answer.
'time' could be 'early' (I've seen this in other clues) and 'early' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for yearly that I've seen before include "Every tenth of a decade" , "At annual intervals" , "on a regular basis" , "Perennial" , "Approximately every 52 weeks" .)