Devout about RE, children of clerics initially matured early (10)
I believe the answer is:
precocious
'early' is the definition.
(similar in meaning)
'devout about re children of clerics initially' is the wordplay.
'devout' becomes 'pious' (devoutly religious).
'about' means one lot of letters goes inside another.
'initially' suggests taking the first letters.
The first letters of 'children of clerics' is 'coc'.
're'+'coc'='recoc'
'pious' going around 'recoc' is 'PRECOCIOUS'.
'matured' acts as a link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for precocious that I've seen before include "Forward" , "ahead in development" , "Early in developing" , "Early-developing" , "early talent" .)