Male in lengthy run in Wagner opera (9)
I believe the answer is:
lohengrin
'wagner opera' is the definition.
(I've seen this before)
'male in lengthy run in' is the wordplay.
'male' becomes 'he' (**).
'in' means one lot of letters goes inside another.
'lengthy' becomes 'long' ('long' can be a synonym of 'lengthy').
'run' becomes 'r' (cricket abbreviation).
'he' inserted into 'long' is 'loheng'.
'loheng'+'r'+'in'='LOHENGRIN'
(Other definitions for lohengrin that I've seen before include "Wagnerian opera" , "Musical entertainment" , "Wagner opera" .)