Usual foul-up as fun can display (5)
I believe the answer is:
snafu
'usual foul-up' is the definition.
'snafu' can be an answer for 'foul-up' (I've seen this in another clue). I'm not certain of the 'usual' bit.
'as fun can display' is the wordplay.
'as fun' anagrams to 'SNAFU'.
Though, I am unsure how this is indicated.
Can you help me to learn more?
(Other definitions for snafu that I've seen before include "mess as usual" , "Customary confusion" , "Chaotic state, mess" , "a mess of things" , "Mess table" .)