### Clue: I am dressing 24, holding up party (5,10)
### Answer: ROBIN GOODFELLOW 
 ### Explanation: Definition: I
 ROBING(dressing;putting on a robe) { [ O(abbrev. for "old";1st part of the answer to 24 across) FELLOW(man;2nd part of the answer to 24 across) ] **containing**(holding) **reversal of**(up, in a down clue) DO(a party;a function) }.
Answer: An alias for the folklore character, Puck, the chosen pseudonym of the setter of this puzzle, the "I" of the definition.