Trouble the setter is going to! (3)
I believe the answer is:
ill
'trouble' is the definition.
(synonyms)
'the setter is going to' is the wordplay.
'the setter' becomes 'i' (the person that set the crossword).
'is going to' becomes 'll' (as in he'll, we'll etc).
'i'+'ll'='ILL'
(Other definitions for ill that I've seen before include "Dicky" , "Sickly" , "Faulty" , "Seasick" , "'Never speak . . . of the dead'" .)