Hi Mikey,
6a - If you reverse/put back the answer you get a slang word meaning "the necessary" (as in 'hand over the necessary - I know, it's not the most obvious meaning)
21a - 5 letter "agent" as in a 'stand in' around/'welcoming' A (from the clue) then 2 letter "swim on vacation" - definition is "fit" (of acute pain)