Rough denture here? What rot! (5,3,7)
I believe the answer is:
under the weather
'rough' is the definition.
(under the weather means feeling unwell or rough)
'denture here? what rot' is the wordplay.
'rot' indicates anagramming the letters (I've seen 'rotting' mean this).
'denture'+'here'+'what'='dentureherewhat'
'dentureherewhat' anagrammed gives 'UNDER THE WEATHER'.
(Other definitions for under the weather that I've seen before include "with clouds gathering overhead" , "Depressed" , "where you have to look for this crossword?" , "not so good" , "Seedy" .)