Fare well, in a word, thanks to volunteers (2-2)
I believe the answer is:
ta-ta
'fare well in a word' is the definition.
The definition suggests a singular noun which matches the answer.
'thanks to volunteers' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'to volunteers' becomes 'TA' (abbreviation for Territorial Army).
'ta'+'ta'='TA-TA'
Can you help me to learn more?
(Other definitions for ta-ta that I've seen before include "Bye-bye" , "Adieu" , "Bye!" , "Cheerio" , "A Farewell" .)