Still single? This creature may not be (4)
I believe the answer is:
yeti
'this creature may not be' is the definition.
(the yeti may not exist)
'still single?' is the wordplay.
'still' becomes 'yet' (synonyms).
'single?' becomes 'i' (one).
'yet'+'i'='YETI'
(Other definitions for yeti that I've seen before include "mythical monster!" , "Your claim to have seen one wouldn't be believed" , "creature never actually seen" , "Legendary beast of the Himalayas?" , "... and creature supposedly there" .)