Taught sound science by repetition (9)
I believe the answer is:
tautology
'repetition' is the definition.
(I have seen 'Unnecessary repetition' mean 'tautology' so perhaps 'repetition' could also mean 'tautology')
'taught sound science by' is the wordplay.
'sound' indicates a 'sounds like' (homophone) clue.
'science' becomes 'ology' (I've seen this before).
'by' means one lot of letters go next to another.
'taught' sounds like 'taut'.
'taut'+'ology'='TAUTOLOGY'
(Other definitions for tautology that I've seen before include "Repetition in different words for a gouty lot" , "identifying myself personally?" , "Use of words repeating something already implied" , "Unnecessary repetition of words" , "as new innovation, perhaps" .)