Flat surface’s length, on the side? (5)
I believe the answer is:
ledge
'flat surface's' is the definition.
(eg a window ledge)
'length on the side?' is the wordplay.
'length' becomes 'l' (abbreviation in geometry).
'on' says to put letters next to each other.
'the side?' becomes 'edge' (edge is a kind of side).
'l'+'edge'='LEDGE'
(Other definitions for ledge that I've seen before include "Mantelpiece" , "Narrow projecting shelf as from window" , "Projecting shelf, horizontal projection" , "Rock shelf" , "rocky situation?" .)