Left, for instance, on one side (3)
I believe the answer is:
leg
'left for' is the definition.
(I know that left is a type of hand)
'instance on one side' is the wordplay.
'instance' becomes 'eg' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' means one lot of letters go next to another.
'one side' becomes 'l' (abbreviation for left).
'eg' after 'l' is 'LEG'.
Can you help me to learn more?
(Other definitions for leg that I've seen before include "Lower limb" , "part of course" , "Gigot" , "Run < on" , "Support; cricket side" .)