Give a warning when backing, also (4)
I believe the answer is:
toot
'give a warning' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'when backing also' is the wordplay.
'when' becomes 't' (I am not sure about this - if you are sure you should believe this answer much more).
'backing' says the letters should be written in reverse.
'also' becomes 'too' (synonyms).
'too' in reverse letter order is 'oot'.
't'+'oot'='TOOT'
Can you help me to learn more?
(Other definitions for toot that I've seen before include "Briefly blow" , "Brief blast on a horn" , "Sound of a car horn" , "Blow a horn" , "Use horn" .)