Worried, agrees to put in an addendum, as one forecasts (8)
I believe the answer is:
presages
'one forecasts' is the definition.
(I've seen this before)
'worried agrees to put in an addendum' is the wordplay.
'worried' indicates an anagram (worry can mean to interfere with or disturb).
'to put in' is an insertion indicator.
'an addendum' becomes 'ps' (I can't explain this - if you can you should give a lot more credence to this answer).
'agrees' anagrammed gives 'resage'.
'resage' going inside 'ps' is 'PRESAGES'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for presages that I've seen before include "Augurs, bodes" , "Foreshadows, is advance sign of" , "Forecasts" , "Foreshadows in press age" .)