Youngster guarded by neurotic Hilda (5)
I believe the answer is:
child
'youngster' is the definition.
('child' can be a synonym of 'youngster')
'guarded by neurotic hilda' is the wordplay.
'guarded by' indicates the answer is hidden within the clue.
'CHILD' is hidden within 'neurotic hilda'.
(Other definitions for child that I've seen before include "Youngster" , "Kid" , "Immature human" , "Son or daughter" , "Little one" .)