Reckon D notice comes first (3)
I believe the answer is:
add
'reckon' is the definition.
(reckon can mean to calculate)
'd notice comes first' is the wordplay.
'notice' becomes 'ad' (short for advert).
'comes first' means one lot of letters go next to another.
'd' after 'ad' is 'ADD'.
(Other definitions for add that I've seen before include "Give more" , "what a computer can do" , "Combine (numbers)" , "Tot; append" , "Say as a further remark" .)