Second story is boring (5)
I believe the answer is:
stale
'boring' is the definition.
(I've seen this before)
'second story' is the wordplay.
'second' becomes 's' (common abbreviation).
'story' becomes 'tale' (synonyms).
's'+'tale'='STALE'
'is' acts as a link.
(Other definitions for stale that I've seen before include "Mouldy or hackneyed" , "Not fresh, being least" , "Lacking freshness" , "Musty or hackneyed" , "Boring" .)