Joy found in single existence (4)
I believe the answer is:
glee
'joy' is the definition.
(I've seen this before)
'in single existence' is the wordplay.
'in' indicates the answer is hidden within the clue.
'GLEE' is hidden within 'single existence'.
'found' acts as a link.
(Other definitions for glee that I've seen before include "Elation" , "Unholy delight" , "Impish enjoyment" , "TV show" , "Sort of club" .)