Tidy small rubbish tip (4)
I believe the answer is:
sort
'tidy' is the definition.
(I've seen this before)
'small rubbish tip' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'rubbish tip' becomes 'ort' (I can't justify this - if you can you should give a lot more credence to this answer).
's'+'ort'='SORT'
Can you help me to learn more?
(Other definitions for sort that I've seen before include "Put in order" , "Type, category" , "Arrange in order" , "Shuffle into desired sequence" , "Tidy" .)