Singer opening in Berlin -- when? First of September (4)
I believe the answer is:
bass
'singer' is the definition.
(bass is a kind of singer)
'opening in berlin when? first of september' is the wordplay.
'opening in' says to take the initial letters.
'when?' becomes 'as' (eg 'when I walked' means 'as I walked').
'first of september' becomes 's' (1st letter of 'september').
The initial letter of 'berlin' is 'b'.
'b'+'as'+'s'='BASS'
(Other definitions for bass that I've seen before include "Large percussion instrument" , "Fish for the low-voiced" , "Lowest male singer" , "Booming percussion instrument" , "Fish; deep voice" .)