Question odd features of ballet during concert (7)
I believe the answer is:
problem
'question' is the definition.
(problem is a kind of question)
'odd features of ballet during concert' is the wordplay.
'odd features of' means one should take alternating letters (letters in odd-numbered positions).
'during' is an insertion indicator.
'concert' becomes 'prom' (abbreviation for promenade concert).
The alternate letters of 'ballet' are 'ble'.
'ble' inserted within 'prom' is 'PROBLEM'.
(Other definitions for problem that I've seen before include "Snag" , "Something that needs solving" , "Matter hard to cope with" , "Hitch" , "Challenging question" .)