Boring; deflated (4)
I believe the answer is:
flat
This clue is a double definition.
'boring' is the first definition.
(flat can mean dull or lifeless)
'deflated' is the second definition.
(thesaurus)
(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" .)