Brainless Sid carrying ram (6)
I believe the answer is:
stupid
'brainless' is the definition.
(synonyms)
'sid carrying ram' is the wordplay.
'carrying' is an insertion indicator.
'ram' becomes 'tup' (both can mean a male sheep).
'sid' placed around 'tup' is 'STUPID'.
(Other definitions for stupid that I've seen before include "Lacking wit or intelligence" , "Obtuse" , "Dim" , "Gormless" , "Imbecilic" .)