Astronomical phenomena: question, when repeated, is about radius (7)
I believe the answer is:
quasars
'astronomical phenomena' is the definition.
(quasar is a type of astronomical phenomenon)
'question when repeated is about radius' is the wordplay.
'question' becomes 'qu' (abbreviation).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'repeated' shows that letters should be duplicated.
'is about' is an insertion indicator.
'radius' becomes 'r' (abbreviation in geometry).
'as' doubled is 'asas'.
'asas' enclosing 'r' is 'asars'.
'qu'+'asars'='QUASARS'
(Another definition for quasars that I've seen is " stars".)