A six-footer's out to hold Sid close by (6)
I believe the answer is:
beside
'close by' is the definition.
'beside' can be an answer for 'by' (I've seen this before). I am unsure of the 'close' bit.
'a six-footer's out to hold sid' is the wordplay.
'a six footer's out' becomes 'bee' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to hold' indicates putting letters inside.
'bee' placed around 'sid' is 'BESIDE'.
Can you help me to learn more?
(Other definitions for beside that I've seen before include "Adjacent to" , "- - the point" , "Next to - compared with" , "Neighbours" , "Next door to" .)