What's regularly clean and then wet again? (7)
I believe the answer is:
laundry
'wet again?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'what's regularly clean and then' is the wordplay.
'what's regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'and then' becomes 'undry' (I can't explain this - if you can you should give a lot more credence to this answer).
The alternate letters of 'clean' are 'la'.
'la'+'undry'='LAUNDRY'
(Other definitions for laundry that I've seen before include "Garments for washing etc." , "Washhouse" , "Where things get washed and ironed" , "Place for washing clothes" , "Dirty washing" .)