Forlorn, hollow dummy covers behind (6)
I believe the answer is:
dreary
'forlorn' is the definition.
(thesaurus)
'hollow dummy covers behind' is the wordplay.
'hollow' means to remove the middle letters (the word is hollowed out).
'covers' indicates putting letters inside.
'behind' becomes 'rear' (both can mean one's bottom).
'dummy' with its centre removed is 'dy'.
'dy' placed around 'rear' is 'DREARY'.
(Other definitions for dreary that I've seen before include "Dismal, funereal" , "Drab and depressing" , "Drab and dismal" , "Dismal, boring" , "Gloomy, boring" .)