Spy on Frenchman before a battle (7)
I believe the answer is:
magenta
'battle' is the definition.
(Magenta is an example)
'spy on frenchman before a' is the wordplay.
'spy' becomes 'agent' (secret agent).
'on' means one lot of letters go next to another.
'frenchman' becomes 'm' (abbreviation for Monsieur).
'before' is a charade indicator (letters next to each other).
'agent' put after 'm' is 'magent'.
'magent'+'a'='MAGENTA'
(Other definitions for magenta that I've seen before include "puce" , "French victory" , "Bright purplish red or magnate" , "Colourful" , "Colour named after a bloody 1859 battle in Italy" .)