Start carping about bad behaviour with crack (7)
I believe the answer is:
crevice
'crack' is the definition.
(I know that crack can be written as crevice)
'start carping about bad behaviour' is the wordplay.
'start' says to take the initial letters.
'about' becomes 're' ('re' means regarding or about).
'bad behaviour' becomes 'vice'.
The initial letter of 'carping' is 'c'.
'c'+'re'+'vice'='CREVICE'
'with' acts as a link.
(Other definitions for crevice that I've seen before include "Narrow crack" , "Split" , "Crack or fissure." , "Narrow rock fissure" , "Opening" .)