Stir up terrible evil when there's no love lost (5)
I believe the answer is:
liven
'stir up' is the definition.
(thesaurus)
'there's no love lost' is the wordplay.
'there's' becomes 'live' (I can't justify this - if you can you should believe this answer much more).
'love' becomes 'o' (love means zero in tennis).
'lost' indicates named letters should be taken away.
'no' with 'o' taken out is 'n'.
'live'+'n'='LIVEN'
'terrible evil when' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for liven that I've seen before include "Invigorate" , "Cheer up" , "Make active" , "Animate, make energetic" , "Cause excitement" .)