My son sent back bogus musical work (8)
I believe the answer is:
symphony
'musical work' is the definition.
(I've seen this before)
'my son sent back bogus' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'sent back' says the letters should be written backwards.
'bogus' becomes 'phony' (synonyms).
'my'+'s'='mys'
'mys' in reverse letter order is 'sym'.
'sym'+'phony'='SYMPHONY'
(Other definitions for symphony that I've seen before include "piece of music" , "Eg, the Eroica" , "Composition for an orchestra" , "Musical work" , "See 17" .)