All answers have an extra letter in the word play (and some are 'themed' missing a word for which you leave a blank cell) 30 down: Check the Parisian crab cooked in Will's strong liquor (6) ???A?N 31 down: Contracted delegate to block leaderless Welsh county's slide downhill (6) ???E??
Also, how do you follow the instruction of what to do with the blanks with 4 down/across do you just leave it blank?
Thanks in advance
30 is thematic - how you'd 'check' something off a list + 'the French' + anag [c]RAB IN
31 a Welsh county 'leaderless' = minus the (g) and minus final [t] which is extra - around DEL(egate).