Even I, perhaps, could become so fearful (12)
I believe the answer is:
apprehensive
'fearful' is the definition.
(I've seen this before)
'even i perhaps could become' is the wordplay.
'could become' indicates an anagram.
'even'+'i'+'perhaps'='eveniperhaps'
'eveniperhaps' anagrammed gives 'APPREHENSIVE'.
'so' acts as a link.
(Other definitions for apprehensive that I've seen before include "Feeling anxiety or fear about future events" , "Fearful, anxious" , "Uneasy, anxious" , "Worried" , "Seven happier (anag.)" .)