Pretend an eccentric was boring (9)
I believe the answer is:
trepanned
'was boring' is the definition.
(to trepan is to bore a hole in a skull)
'pretend an eccentric' is the wordplay.
'eccentric' indicates an anagram.
'pretend'+'an'='pretendan'
'pretendan' is an anagram of 'TREPANNED'.
(Other definitions for trepanned that I've seen before include "cut in the head" , "Holed" , "performed outdated surgery" .)