The majority long to adopt high-class facial growth (9)
I believe the answer is:
moustache
'facial growth' is the definition.
(I've seen this before)
'the majority long to adopt high-class' is the wordplay.
'the majority' becomes 'most' (most of a group).
'long' becomes 'ache' (to long for something is to ache for it).
'to adopt' is an insertion indicator (inserted letters are taken in or adopted).
'high class' becomes 'U' (as in U and non-U speech).
'most'+'ache'='mostache'
'mostache' placed around 'u' is 'MOUSTACHE'.
(Other definitions for moustache that I've seen before include "Walrus, e.g." , "Food trap" , "something hairy" , "Handlebar or walrus, for example" , "whiskers" .)