Describe tartan design as silly (14)
I believe the answer is:
scatterbrained
'silly' is the definition.
(thesaurus)
'describe tartan design' is the wordplay.
'design' indicates anagramming the letters.
'describe'+'tartan'='describetartan'
'describetartan' anagrammed gives 'SCATTERBRAINED'.
'as' is the link.
(Other definitions for scatterbrained that I've seen before include "silly" , "Disorganised, lacking concentration" , "not together" , "Disorganised and unable to concentrate" , "Inattentive" .)