Boys were developing facial hair (8)
I believe the answer is:
eyebrows
'hair' is the definition.
(I know that eyebrows is a type of human facial hair)
'boys were developing' is the wordplay.
'developing' is an anagram indicator (letters developed into a new form).
'boys'+'were'='boyswere'
'boyswere' anagrammed gives 'EYEBROWS'.
'facial' is the link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for eyebrows that I've seen before include "part of one's features" , "these may have been plucked for beauty" , "that may be surprisingly elevated" .)