Rubbish deposited by a vale (2-2)
I believe the answer is:
ta ta
'vale' is the definition.
(ta-ta can mean bye - 'vale' is farewell in Latin)
'rubbish deposited by a' is the wordplay.
'rubbish' becomes 'tat' (low-quality items or material).
'deposited by' says to put letters next to each other.
'tat'+'a'='TA-TA'
(Other definitions for ta ta that I've seen before include "Informal and repetitive goodbye" , "Cheerio" , "Bye!" , "Adieu" , "Goodbye (colloq.)" .)