Really enjoyed this, and 6d is also my COD - very clever!
I can't parse the second word of 4d though.
Barry, re 1a; The first word, plus the first letter of the second word, is a sort of jokey word for people who do sums (it's more obvious than you'd think) and the rest of the second word is an anagram of 2 words in the clue. For definition, think seasonal.