The Moirai start fearing a setback (5)
I believe the answer is:
fates
'the moirai' is the definition.
(thesaurus)
'start fearing a setback' is the wordplay.
'start' suggests taking the first letters.
'setback' becomes 'tes' (I've seen this in other clues).
The initial letter of 'fearing' is 'f'.
'f'+'a'+'tes'='FATES'
(Other definitions for fates that I've seen before include "Lots" , "Three Greek goddesses who presided over life and death" , "Mythological goddesses controlling destinies" , "Three Greek goddesses who presided over people's destinies" , "Goddesses guiding human lives" .)