Dispose of bath thrown in gully (8)
I believe the answer is:
dispatch
'dispose of' is the definition.
(thesaurus)
'bath thrown in gully' is the wordplay.
'bath' becomes 'spa' (I've seen this before).
'thrown in' means one lot of letters goes inside another.
'gully' becomes 'ditch'.
'spa' inserted into 'ditch' is 'DISPATCH'.
(Other definitions for dispatch that I've seen before include "report I preferred" , "Send promptly; murder" , "Communication" , "Send off (mail)" , "Send off, goods perhaps" .)