Opening chapter about immoral behaviour (7)
I believe the answer is:
crevice
'opening' is the definition.
(I know that crevice is a type of opening)
'chapter about immoral behaviour' is the wordplay.
'chapter' becomes 'c' (abbreviation).
'about' becomes 're' ('re' means regarding or about).
'immoral behaviour' becomes 'vice' (I've seen this before).
'c'+'re'+'vice'='CREVICE'
(Other definitions for crevice that I've seen before include "Narrow crack" , "Opening" , "Cleft, fissure" , "Split" , "Narrow rock fissure" .)