Become stupid, briefly, like Frankenstein? (6)
I believe the answer is:
gothic
'like frankenstein?' is the definition.
(I've seen this in another clue)
'become stupid briefly' is the wordplay.
'become' becomes 'go' (eg to go rotten is to become rotten).
'stupid' becomes 'thick' (similar in meaning).
'briefly' means to remove the last letter.
'thick' with its final letter taken away is 'thic'.
'go'+'thic'='GOTHIC'
(Other definitions for gothic that I've seen before include "Style of architecture, literatureand film" , "Style between Romanesque and Classical" , "Style of mediaeval architecture" , "as some Germans were" , "Style of architecture popular in Europe in the 12th to 16th centuries" .)