Frenchman embroiled in impressive battle (5)
I believe the answer is:
somme
'battle' is the definition.
(Battle of the Somme in WW1)
'frenchman embroiled in impressive' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur).
'embroiled in' means one lot of letters goes inside another.
'impressive' becomes 'some' (I can't explain this - if you can you should give a lot more credence to this answer).
'm' put into 'some' is 'SOMME'.
Can you help me to learn more?
(Other definitions for somme that I've seen before include "French 2 down" , "European river" , "bloody fight" , "World War I battlefield around French river" , "World War one battle where great losses were incurred" .)