Dim Greek character captured by horse (6)
I believe the answer is:
stupid
'dim' is the definition.
(similar in meaning)
'greek character captured by horse' is the wordplay.
'greek character' becomes 'pi' (letter from the Greek alphabet).
'captured by' means one lot of letters goes inside another.
'horse' becomes 'stud'.
'pi' going inside 'stud' is 'STUPID'.
(Other definitions for stupid that I've seen before include "dope" , "Idiot" , "Not very bright" , "Not sensible or intelligent" , "Barmy" .)