Being sixteen involved acne on a regular basis (9)
I believe the answer is:
existence
'being' is the definition.
('existence' can be a synonym of 'being')
'sixteen involved acne on a regular basis' is the wordplay.
'involved' is an anagram indicator.
'on a regular basis' means one should take alternating letters.
The alternating letters of 'acne' are 'ce'.
'sixteen' anagrammed gives 'existen'.
'existen'+'ce'='EXISTENCE'
(Other definitions for existence that I've seen before include "survival" , "The business of being" , "State of being" , "A way of living" , "Way of life" .)