Strange and unconventional behaviour (12)
I believe the answer is:
eccentricity
'strange and unconventional behaviour' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for eccentricity that I've seen before include "Slight strangeness (of character)" , "feature of orbit?" , "Card displays this" , "Unconventional nature" , "Strange and unconventional behaviour" .)