Fess up (5)
'fess up' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for admit that I've seen before include "Recognise" , "Profess" , "Agree to the truth" , "Stop being obstructive" , "Reveal" .)
(Another definition for admitit that I've seen is " demand made by 11 [PRIVATE EYE] ?".)