Sarcastic bravo to the first person with metal headgear (6)
I believe the answer is:
biting
'sarcastic' is the definition.
(similar in meaning)
'bravo to the first person with metal headgear' is the wordplay.
'bravo' becomes 'b' (phonetic alphabet: alpha, bravo, charlie etc.).
'to the first person' becomes 'I' (first-person pronoun).
'with metal headgear' becomes 'ting' (I can't justify this - if you can you should believe this answer much more).
'b'+'i'+'ting'='BITING'
Can you help me to learn more?
(Other definitions for biting that I've seen before include "(Of wit) harsh, cruel" , "Mordant" , "Keen" , "Painful or stinging" , "Sarcastic; very cold" .)