Smite ogre occasionally participating in attack on fortress (5)
I believe the answer is:
siege
'participating in attack on fortress' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'smite ogre occasionally' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'smite'+'ogre'='smiteogre'
The alternating letters of 'smiteogre' are 'SIEGE'.
Can you help me to learn more?
(Other definitions for siege that I've seen before include "Besetting" , "Offence in which fortified place is surrounded" , "Operation to surround and cut off supplies" , "Act of surrounding a town or fortress" , "Isolating attack" .)