Joke puts pressure on peacekeepers (3)
I believe the answer is:
'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).
'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" .)