Soldier heralds the French monarch, playing horn (6)
I believe the answer is:
antler
'playing horn' is the definition.
'antler' can be an answer for 'horn' (antler is a kind of horn). I'm unsure of the 'playing' bit.
'soldier heralds the french monarch' is the wordplay.
'soldier heralds' becomes 'ant' (I can't explain this - if you can you should believe this answer much more).
'the french' becomes 'le' ('the' in French).
'monarch' becomes 'R' (abbreviation for regina, e.g. in ER ).
'ant'+'le'+'r'='ANTLER'
Can you help me to learn more?
(Other definitions for antler that I've seen before include "Branched animal horn" , "Animal's extension" , "Deer's branching horn" , "Branch" , "Stag's horn" .)