It's seen essentially on faces to effect rejuvenation (5)
I believe the answer is:
toner
'effect rejuvenation' is the definition.
The definition suggests a singular noun which matches the answer.
'it's seen essentially on faces' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'seen essentially on faces' becomes 'oner' (I am not sure about this - if you are sure you should believe this answer much more).
't'+'oner'='TONER'
'to' is the link.
Can you help me to learn more?
(Other definitions for toner that I've seen before include "Astringent liquid (to improve skin)" , "Lotion for cleansing the skin" , "Copier powder" , "Skin-cleansing lotion" , "Black or coloured powder used in a printer" .)