Horizontal — dull (4)
I believe the answer is:
flat
I believe this clue is a double definition.
'horizontal' is the first definition.
(similar in meaning)
'dull' is the second definition.
(synonyms)
(Other definitions for flat that I've seen before include "Dull; apartment" , "Even -- pitched too low" , "one or more in uniform" , "Level - below the natural note" , "Unnatural - uninteresting - apartment" .)