Crow about French XI, second in league for period (6,3)
I believe the answer is:
bronze age
'period' is the definition.
(I know that Bronze Age is a period)
'crow about french xi second in league' is the wordplay.
'crow' becomes 'brag' (synonyms).
'about' means one lot of letters goes inside another.
'french xi' becomes 'onze' ('eleven' in French).
'second in league' becomes 'e' (2nd letter of 'league').
'brag' placed around 'onze' is 'bronzeag'.
'bronzeag'+'e'='BRONZE AGE'
'for' acts as a link.
(Other definitions for bronze age that I've seen before include "Years long ago" , "Period of early history" , "remarkable culture" , "many years" , "Period between the Stone and Iron eras" .)