A revolutionary painting is call to arms (6)
I believe the answer is:
alarum
'call to arms' is the definition.
I can't tell whether this definition defines the answer.
'a revolutionary painting' is the wordplay.
'revolutionary' shows that the letters should be reversed in order.
'painting' becomes 'mural' (mural is a kind of painting).
'mural' back-to-front is 'larum'.
'a'+'larum'='ALARUM'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for alarum that I've seen before include "commotion" , "Warning sound" , "Warning bell (archaic)" , "conflict" , "disturbance" .)