Sum's unlikely with unknown quantity (5)
I believe the answer is:
tally
'sum's' is the definition.
('tally' can be a synonym of 'sum')
'unlikely with unknown quantity' is the wordplay.
'unlikely' becomes 'tall' (I've seen this in other clues).
'with' says to put letters next to each other.
'unknown quantity' becomes 'y' (algebra).
'tall'+'y'='TALLY'
(Other definitions for tally that I've seen before include "Match - score" , "Count, add up" , "Count up; match" , "Running total" , "Mark" .)