Joke puts pressure on peacekeepers (3)
I believe the answer is:
pun
'joke' is the definition.
(pun is a type of joke)
'pressure on peacekeepers' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'on' says to put letters next to each other.
'peacekeepers' becomes 'UN' (United Nations, peacekeeping organisation).
'p'+'un'='PUN'
'puts' acts as a link.
(Other definitions for pun that I've seen before include "A play on words - not the height of wit" , "A play on words, not really wit" , "joke" , "Humorous wordplay" , "A play on words, perhaps the lowest form of wit" .)