Beside all but Sid, a six-footer (3)
I believe the answer is:
bee
'a six-footer' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'beside all but sid' is the wordplay.
'beside' becomes 'by'.
'all but' means to remove the last letter ('all but' can mean 'almost' or 'nearly').
'sid' becomes 'ee' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'by' with its last letter taken off is 'b'.
'b'+'ee'='BEE'
Can you help me to learn more?
(Other definitions for bee that I've seen before include "Apian insect" , "Social worker possibly" , "Honey generator" , "Insect that bumbles?" , "In one's bonnet, it's a persistent idea" .)