Gladstone's son (3)
I believe the answer is:
lad
'son' is the definition.
(lad can mean a boy or son)
'gladstone's' is the wordplay.
'LAD' is hidden within the letters.
However, I'm not sure how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for lad that I've seen before include "Stable employee" , "Son" , "Boy, familiar" , "(Housman's Shropshire) boy" , "Boy or youth" .)