Strange and unconventional behaviour (12)
I believe the answer is:
eccentricity
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">strange and unconventional behaviour</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)