Jack, one child in five with a classic name (7)
I believe the answer is:
tarquin
'five with a classic name' is the definition.
I can't tell whether this defines the answer.
'jack one child' is the wordplay.
'jack' becomes 'tar' (both are informal terms for sailor).
'one child' becomes 'quin' (I have seen 'Child of five ' mean 'quin' so perhaps 'child' could also mean 'quin').
'tar'+'quin'='TARQUIN'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for tarquin that I've seen before include "Last Etruscan king of Rome, 534-510 BC" , "proud king" .)