Unusual end to war - a run on energy (4)
I believe the answer is:
rare
'unusual' is the definition.
(thesaurus)
'end to war a run on energy' is the wordplay.
'end to' suggests the final letters.
'a run' becomes 'ar' (I've seen this in other clues**).
'on' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mcr).
The final letter of 'war' is 'r'.
'r'+'ar'+'e'='RARE'
(Other definitions for rare that I've seen before include "Not found often" , "like Ben Jonson, Jack Young thought" , "Seldom seen or met" , "Meat that's lightly cooked" , "difficult to find?" .)