This person's going to trouble (3)
I believe the answer is:
ill
'trouble' is the definition.
(synonyms)
'this person's going to' is the wordplay.
'this person' becomes 'I'.
'going to' becomes 'll' (as in he'll, we'll etc).
'i'+'ll'='ILL'
(Other definitions for ill that I've seen before include "Unhealthy" , "Out of sorts" , "weak" , "Shakespeare: '. . . met by moonlight'" , "Evil or sick" .)