Son bitter about A levels (6)
I believe the answer is:
strata
'levels' is the definition.
(I've seen this before)
'son bitter about a' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'bitter' becomes 'tart' (tart can mean bitter-tasting).
'about' says the letters should be written backwards.
'tart' in reverse letter order is 'trat'.
's'+'trat'+'a'='STRATA'
(Other definitions for strata that I've seen before include "Layers into which rocks are divided" , "Layers of sedimentary rock" , "Parallel layers" , "Geological levels" , "levels of society" .)