Parent keeps naughty girl finally in line for many years (10)
I believe the answer is:
millennium
'many years' is the definition.
(a thousand years)
'parent keeps naughty girl finally in line' is the wordplay.
'parent' becomes 'mum' (mum is a kind of parent).
'keeps' means one lot of letters goes inside another.
'naughty' indicates an anagram.
'finally' suggests the final letters.
The final letter of 'girl' is 'l'.
'l'+'in'+'line'='linline'
'linline' with letters rearranged gives 'illenni'.
'mum' going around 'illenni' is 'MILLENNIUM'.
'for' is the link.
(Other definitions for millennium that I've seen before include "Period of one thousand years" , "1,000 years" , "age" .)