Overhaul extremely ramshackle coupling (6)
I believe the answer is:
repair
'overhaul' is the definition.
(I know that overhaul can be written as repair)
'extremely ramshackle coupling' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'coupling' becomes 'pair' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ramshackle' with its centre taken out is 're'.
're'+'pair'='REPAIR'
Can you help me to learn more?
(Other definitions for repair that I've seen before include "Maintain" , "Proceed - restore" , "Get back into good shape" , "Withdraw" , "Go - fix" .)