Reckon D notice should come first (3)
I believe the answer is:
add
'reckon' is the definition.
(reckon can mean to calculate)
'd notice should come first' is the wordplay.
I cannot quite understand how this works, but
'd' is within the answer.
'notice' could be 'ad' (short for advert) and 'ad' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for add that I've seen before include "Also" , "put two and two together" , "problem for hyperactive children" , "Aggregate or supplement" , "Determine the sum of" .)