Curse being brainier, oddly (4)
I believe the answer is:
bane
'curse' is the definition.
(synonyms)
'brainier oddly' is the wordplay.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
The alternate letters of 'brainier' are 'BANE'.
'being' is the link.
(Other definitions for bane that I've seen before include "Cause of trouble or worry, like bean" , "Blight" , "scourge" , "Cause of trouble or anxiety" , "Cause of distress, ruin" .)