Following peacekeepers is amusing (3)
I believe the answer is:
fun
'is amusing' is the definition.
(synonyms)
'following peacekeepers' is the wordplay.
'following' becomes 'f' (used when citing page numbers etc).
'peacekeepers' becomes 'UN' (United Nations, peacekeeping organisation).
'f'+'un'='FUN'
(Other definitions for fun that I've seen before include "Light-hearted amusement, jollity" , "'Merriment, playfulness (3)'" , "entertainment" , "Playful amusement" , "Light-hearted pleasure" .)