Again going into that is an indication of possible evil to come (6)
I believe the answer is:
threat
'an indication of possible evil to come' is the definition.
I can't judge whether this definition defines the answer.
'again going into that' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'going into' means one lot of letters goes inside another.
're' inserted inside 'that' is 'THREAT'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for threat that I've seen before include "Intimidatory words to exact compliance" , "Impending danger" , "Statement of intention to harm" , "Indication of danger" , "Potential source of danger" .)