Lad ultimately concealed no child (5)
I believe the answer is:
adult
'no child' is the definition.
(I've seen this before)
'lad ultimately concealed' is the wordplay.
'concealed' says the answer is hidden in the clue.
'ADULT' can be found hidden inside 'lad ultimately'.
(Other definitions for adult that I've seen before include "Supposedly "responsible" person" , "Mature, fully-grown" , "one no longer in minority" , "Blue" , "(Of an animal) fully developed" .)