Two A-levels I’d failed? A fanciful story (3,5,4)
I believe the answer is:
old wives tale
'a fanciful story' is the definition.
'two a-levels i'd failed?' is the wordplay.
'failed?' indicates anagramming the letters.
'two'+'a'+'levels'+'id'='twoalevelsid'
'twoalevelsid' anagrammed gives 'OLD WIVES' TALE'.
(Other definitions for old wives tale that I've seen before include "Widely-held but incorrect belief" , "Superstitious or misleading traditional belief" , "Inaccurate tradition" , "Belief, often erroneous, passed on by word of mouth" , "Myth (by Arnold Bennett?)" .)