Son raised to protect younger one’s sweet but thick (5)
I believe the answer is:
syrup
'one's sweet but thick' is the definition.
'son raised to protect younger' is the wordplay.
'son' becomes 's'.
'raised' becomes 'up' ('up' can be similar in meaning to 'raised').
'to protect' means one lot of letters goes inside another.
'younger' becomes 'yr' (abbreviation used in some aristocratic titles).
's'+'up'='sup'
'sup' enclosing 'yr' is 'SYRUP'.
(Other definitions for syrup that I've seen before include "cough mixture?" , "Viscous liquid" , "Sticky stuff" , "Sugary substance" , "Treacle" .)