Blunt old PM collecting key cake ingredient (7,4)
I believe the answer is:
candied peel
'cake ingredient' is the definition.
'blunt old pm collecting key' is the wordplay.
'blunt old' becomes 'candid' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pm' becomes 'peel' (I've seen this before).
'collecting' is an insertion indicator.
'key' becomes 'e' (I've seen this in other clues).
'candid'+'peel'='candidpeel'
'candidpeel' placed around 'e' is 'CANDIED PEEL'.
(Another definition for candied peel that I've seen is " topping!".)