Is put off by a day, which is silly (6)
I believe the answer is:
stupid
'is silly' is the definition.
(I've seen this before)
'is put off by a day' is the wordplay.
'off' indicates anagramming the letters.
'by' is a charade indicator (letters next to each other).
'a day' becomes 'd' (abbreviation for day).
'is'+'put'='isput'
'isput' is an anagram of 'stupi'.
'stupi'+'d'='STUPID'
'which' is the link.
(Other definitions for stupid that I've seen before include "Idiotic" , "Senseless" , "Asinine" , "Not sensible or intelligent" , "(Seeming to be) lacking intelligence" .)