Alumnus has extremely obscure pipe (4)
I believe the answer is:
oboe
'pipe' is the definition.
(thesaurus)
'alumnus has extremely obscure' is the wordplay.
'alumnus' becomes 'ob' (old boy, as in an old private school pupil).
'has' is a charade indicator (letters next to each other).
'extremely' says to hollow out the word (remove centre letters) (letters on the extremes of the word are used).
'obscure' with its middle taken out is 'oe'.
'ob'+'oe'='OBOE'
(Other definitions for oboe that I've seen before include "music producer" , "it's blown" , "Double-reeded woodwind instrument" , "Slender double-reed instrument" , "Double-reed wind instrument" .)