Royal Society dons disclose change in direction (8)
I believe the answer is:
reversal
'change in direction' is the definition.
(I've seen this before)
'royal society dons disclose' is the wordplay.
'royal society' becomes 'RS'.
'dons' indicates putting letters inside (don means to wear or put on).
'disclose' becomes 'reveal' (synonyms).
'rs' put into 'reveal' is 'REVERSAL'.
(Other definitions for reversal that I've seen before include "that makes no difference to mum or dad" , "Set back" , "Volte-face" , "A turning backwards" , "A change to an opposite direction" .)