Soloist not even opening (4)
I believe the answer is:
slit
'opening' is the definition.
(slit is a kind of opening)
'soloist not even' is the wordplay.
'not even' means one should take alternating letters (remove even-numbered letters).
The alternating letters of 'soloist' are 'SLIT'.
(Other definitions for slit that I've seen before include "Narrow rent" , "Long straight cut" , "Long narrow opening" , "Narrow straight cut or opening" , "Narrow aperture" .)