Once the greatest issue for her (6)
I believe the answer is:
alison
'once the greatest' is the definition.
I can't judge whether this defines the answer.
'issue for her' is the wordplay.
'issue' becomes 'son' (issue can mean someone's offspring).
'for' means one lot of letters go next to another (I've seen this in other clues).
'her' becomes 'ali' ().
'son' after 'ali' is 'ALISON'.
Can you help me to learn more?
(Other definitions for alison that I've seen before include "lady" , "Girl" , "female" , "Girl's name" , "woman" .)