Long skirt on mother is the greatest! (7)
I believe the answer is:
maximum
'the greatest' is the definition.
(I've seen this before)
'long skirt on mother' is the wordplay.
'long skirt' becomes 'maxi' (I've seen this before).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'mother' becomes 'mum' (informal term for mother).
'maxi'+'mum'='MAXIMUM'
'is' is the link.
(Other definitions for maximum that I've seen before include "Greatest amount" , "Largest in group of numbers" , "Greatest possible quantity" , "The largest possible quantity or degree" , "Utmost" .)