To sum up, less than a morsel to eat (6)
I believe the answer is:
mussel
'a morsel to eat' is the definition.
I can't tell whether this definition defines the answer.
'to sum up less' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'less' becomes 'sel' (I can't justify this - if you can you should give a lot more credence to this answer).
'sum' back-to-front is 'mus'.
'mus'+'sel'='MUSSEL'
'than' is the link.
Can you help me to learn more?
(Other definitions for mussel that I've seen before include "Black marine bivalve" , "Shellfish sounds like part of the body" , "Edible marine mollusc sold by Molly Malone - not the cockle" , "Edible bivalve mollusc, common in sea" , "Seafood item" .)