Head boy initially introduces rule over headphones (5)
I believe the answer is:
brain
'head' is the definition.
(head can mean someone's brain or intellect)
'boy initially introduces rule over headphones' is the wordplay.
'boy initially' becomes 'b' (I am not sure about this - if you are sure you should believe this answer much more).
'introduces' means one lot of letters go next to another (some letters come before or introduce others).
'rule' becomes 'rein' (synonyms).
'over headphones' indicates a 'sounds like' (homophone) clue (as it might sound through headphones).
'rein' is a homophone of 'rain'.
'b'+'rain'='BRAIN'
Can you help me to learn more?
(Other definitions for brain that I've seen before include "Clever type" , "Intelligence" , "Encephalon" , "Thinking thing" , "to get hit over the head" .)