In outer Clapham, there needs to be an open space (5)
I believe the answer is:
chasm
'space' is the definition.
(chasm is a kind of space)
'in outer clapham there' is the wordplay.
'in' indicates putting letters inside.
'outer' suggests removing the centre.
'there' becomes 'has' (I can't explain this - if you can you should give a lot more credence to this answer).
'clapham' with its centre taken out is 'cm'.
'cm' enclosing 'has' is 'CHASM'.
'needs to be an open' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for chasm that I've seen before include "Wide difference" , "Pit" , "big break?" , "Wide gap" , "Yawning gap" .)