Boring; deflated (4)
I believe the answer is:
flat
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">boring</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(flat can mean dull or lifeless)</span><br/ ><br/ >'<span class="explanation-format__original">deflated</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ ></span>
(Other definitions for flat that I've seen before include "Uninteresting - piece of scenery" , "Accommodation < still" , "Even -- pitched too low" , "like some races" , "Dull; set of rooms" .)