To sum up, possibly less shellfish (7)
I believe the answer is:
mussels
'shellfish' is the definition.
(I've seen this before)
'to sum up possibly less' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'sum' is 'mus' which is located in the answer.
an anagram of 'less' is 'sels' which is within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for mussels that I've seen before include "Shellfish" , "Black marine bivalves" .)