Excessive enthusiasm (5)
I believe the answer is:
mania
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">excessive enthusiasm</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 mania that I've seen before include "wildness" , "Over-enthusiasm" , "fixation" , "A (violent) mood disorder" , "Mental disorder that results in excessive or violent responses" .)