Letter, strangely, is open about student (7)
I believe the answer is:
epsilon
'letter' is the definition.
(epsilon is a kind of letter)
'strangely is open about student' is the wordplay.
'strangely' indicates anagramming the letters.
'about' is an insertion indicator.
'student' becomes 'l' (as in L-plates for learner drivers).
'is'+'open'='isopen'
'isopen' is an anagram of 'epsion'.
'epsion' enclosing 'l' is 'EPSILON'.
(Other definitions for epsilon that I've seen before include "small number mispronounced, almost completely the wrong way round" , "character in Greece" , "Greek short E" , "Greek character" , "5th letter of the Greek alphabet" .)