Left, for instance, on one side (3)
I believe the answer is:
leg
'left' is the definition.
(left leg)
'for instance on one side' is the wordplay.
'for instance' becomes 'eg' (short for 'exempli gratia', 'for example' in Latin).
'on' means one lot of letters go next to another.
'one side' becomes 'l' (abbreviation for left).
'eg' put after 'l' is 'LEG'.
(Other definitions for leg that I've seen before include "Lower limb" , "part of course" , "Gigot" , "Run < on" , "Support; cricket side" .)