The side of the legs? (6)
I believe the answer is:
eleven
I believe this clue is a double definition.
'the side' is the first definition.
(I've seen this before)
'legs?' is the second definition.
(eleven is 'legs eleven' in the game bingo)
'of the' acts as a link.
(Other definitions for eleven that I've seen before include "Lowest two-digit prime" , "Sporting side" , "Late" , "Enough for a soccer side" , "XI as Roman numeral" .)