Square model entirely unknown (5)
I believe the answer is:
tally
'square' is the definition.
(I know that square is a more specific form of the action tally)
'model entirely unknown' is the wordplay.
'model' becomes 'T' (Ford Model T car).
'entirely' becomes 'all' (all can mean entirely or completely).
'unknown' becomes 'y' (algebra).
't'+'all'+'y'='TALLY'
(Other definitions for tally that I've seen before include "Match - score" , "Coincide" , "Agree - count" , "Square" , "Count up; match" .)