When rude theatre plays becoming seedy (5,3,7)
I believe the answer is:
under the weather
'seedy' is the definition.
(I've seen this before)
'when rude theatre plays' is the wordplay.
'plays' indicates anagramming the letters.
'when'+'rude'+'theatre'='whenrudetheatre'
'whenrudetheatre' is an anagram of 'UNDER THE WEATHER'.
'becoming' acts as a link.
(Other definitions for under the weather that I've seen before include "Sickening" , "the worse for wear" , "Sick" , "Seedy" , "with clouds gathering overhead" .)