Prime Minister's penultimate press inquiry? Not so (6)
I believe the answer is:
eleven
'prime' is the definition.
(I've seen this before)
'minister's penultimate press inquiry? not so' is the wordplay.
'minister's penultimate' becomes 'e' (I can't explain this - if you can you should believe this answer much more).
'press inquiry?' becomes 'leveson' (the Leveson Inquiry into the UK press).
'not' suggests deleting specific letters.
'leveson' with 'so' taken away is 'leven'.
'e'+'leven'='ELEVEN'
(Other definitions for eleven that I've seen before include "Not first team (6,6) - See 24" , "Cricketing side" , "Number killed in the Peterloo massacre" , "Number of pipers piping" , "Number on a side (6)" .)