Left inside so germs spread, which is stupid (8)
I believe the answer is:
gormless
'is stupid' is the definition.
(similar in meaning)
'left inside so germs spread' is the wordplay.
'left' becomes 'l' (common abbreviation).
'inside' is an insertion indicator.
'spread' indicates anagramming the letters.
'so'+'germs'='sogerms'
'sogerms' is an anagram of 'gormess'.
'l' going inside 'gormess' is 'GORMLESS'.
'which' is the link.
(Other definitions for gormless that I've seen before include "Foolish, stupid" , "Stupid (slang)" , "Stupid - vacant" , "Slow" , "Lacking the sense one was born with" .)