Become endlessly stupid, and like horror stories (6)
I believe the answer is:
gothic
'like horror stories' is the definition.
'become endlessly stupid' is the wordplay.
'become' becomes 'go' (synonyms).
'endlessly' means to remove the last letter.
'stupid' becomes 'thick' (similar in meaning).
'thick' with its final letter removed is 'thic'.
'go'+'thic'='GOTHIC'
'and' acts as a link.
(Other definitions for gothic that I've seen before include "Architectural style in Europe 12th-16th centuries" , "Old language" , "Style of architecture - ornate typeface" , "Hit cog badly to find a style of building" , "Style of architecture common in Europe in 12th-16th centuries" .)