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 am not sure about this - if you are sure you should believe this answer much more).
'love' becomes 'o' (love means zero in tennis).
'lost' suggests deleting specific letters.
'no' with 'o' taken out is 'n'.
'live'+'n'='LIVEN'
'terrible evil when' acts as a 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" .)