Charles Manson's first opening? (5)
I believe the answer is:
chasm
'opening?' is the definition.
(I know that chasm is a type of gap)
'charles manson's first' is the wordplay.
'charles' becomes 'Chas' (shortened form).
'manson's first' becomes 'm' (1st letter of 'manson').
'chas'+'m'='CHASM'
(Other definitions for chasm that I've seen before include "Deep opening in the earth's surface" , "Wide gap" , "What's split" , "Deep fissure; gulf" , "Abyss" .)