Crucible girl can sink ball with gravitational effect (7,3)
I believe the answer is:
melting pot
'crucible' is the definition.
(I know that crucible can be written as melting pot)
'girl can sink ball with gravitational effect' is the wordplay.
'girl' becomes 'Mel' (short for Melanie).
'can' becomes 'tin' (both are metal containers).
'sink ball' becomes 'pot' (as in potting a ball in snooker).
'with' says to put letters next to each other.
'gravitational effect' becomes 'G' (gravity).
'pot' put after 'g' is 'gpot'.
'mel'+'tin'+'gpot'='MELTING-POT'
(Other definitions for melting pot that I've seen before include "In which to mix metals, immigrants" , "Crucible'" , "Dissolving container" , "Place where different peoples mix" , "Vessel - place for mixture" .)