Name old prison party - it's not possible (2,3,2)
I believe the answer is:
no can do
'it's not possible' is the definition.
(I've seen this before)
'name old prison party' is the wordplay.
'name' becomes 'n' (common abbreviation e.g. on forms).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'prison' becomes 'can' (the can is American slang for prison).
'party' becomes 'do' (do is a kind of party).
'n'+'o'+'can'+'do'='NO CAN DO'
(Other definitions for no can do that I've seen before include "Absolutely not!" , "Impossible, I'm afraid!" , "Informal expression of refusal" , "Bring-a-bottle party?" , "That's not possible" .)