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.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'possibly' indicates an anagram.
'sum' written backwards gives 'mus'.
'less' is an anagram of 'sels'.
'mus'+'sels'='MUSSELS'
(Other definitions for mussels that I've seen before include "Shellfish" , "Black marine bivalves" .)