Courteous petty officer set fire to evil leader (6)
I believe the answer is:
polite
'courteous' is the definition.
(similar in meaning)
'petty officer set fire to evil leader' is the wordplay.
'petty officer' becomes 'po' (naval abbreviation).
'set fire to' becomes 'lit' (as in lighting a fire).
'leader' indicates taking the first letters.
The initial letter of 'evil' is 'e'.
'po'+'lit'+'e'='POLITE'
(Other definitions for polite that I've seen before include "Minding one's p's and q's" , "Considerate, cultured" , "Well-behaved" , "Not rude" , "Having good manners, socially correct" .)