He serves to denote a hairstyle (7)
I believe the answer is:
pageboy
'a hairstyle' is the definition.
(pageboy is a kind of hairstyle)
'he serves to denote' is the wordplay.
I cannot quite see how this works, but
'he' could be 'page' and 'page' is found in the answer.
The remaining letters 'boy' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for pageboy that I've seen before include "Lad at wedding" , "Youthful hotel attendant" , "train supporter" , "Young bridal attendant" , "Bride's attendant" .)