Witty fellow covering part of leg for cleaning (7)
I believe the answer is:
washing
'cleaning' is the definition.
(washing is a kind of cleaning)
'witty fellow covering part of leg' is the wordplay.
'witty fellow' becomes 'wag' (I can't explain this - if you can you should believe this answer much more).
'covering' means one lot of letters goes inside another.
'part of leg' becomes 'shin' ('shin' is part of 'leg').
'wag' placed around 'shin' is 'WASHING'.
'for' is the link.
Can you help me to learn more?
(Other definitions for washing that I've seen before include "Cleaning with soap and water" , "Cleaning - laundry" , "Cleansing" , "(and 18) Domestic appliance" .)