Food for spineless lot in old assembly (4,2,5)
I believe the answer is:
diet of worms
'old assembly' is the definition.
(1521 imperial assembly held at Worms in Germany)
'food for spineless lot' is the wordplay.
I cannot quite see how this works, but
'food' could be 'diet' (diet is a kind of food) and 'diet' is found in the answer.
'lot' could be 'm' and 'm' is found in the answer.
This explanation may well be incorrect...
'in' is the link.
(Other definitions for diet of worms that I've seen before include "habitual early birds might enjoy?" , "Assembly trying Luther in 1521" , "Council meeting" , "that Luther fled" , "It declared Martin Luther an outlaw" .)