Some silica gel essence makes you eternally young (7)
I believe the answer is:
ageless
'eternally young' is the definition.
(I've seen this before)
'some silica gel essence' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'AGELESS' is hidden within 'silica gel essence'.
'makes you' acts as a link.
(Other definitions for ageless that I've seen before include "Not growing older" , "unaffected by time" , "Always youthful" , "Never growing old" , "Sea-legs (anag.)" .)