Hold-up by accessory? This might boost porridge dished out
'this might boost porridge dished out' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'hold-up by accessory?' is the wordplay.
'hold-up' becomes 'jam' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'by' says to put letters next to each other.
'accessory?' becomes 'extra' (I've seen this before).
'jam' after 'extra' is 'EXTRA JAM'.
Can you help me to learn more?
(Another definition for extra jam that I've seen is " Preserve following regulation".)