Small volume about one pursuing girl is a masterpiece (7)
I believe the answer is:
Here is my best explanation:
'a masterpiece' is the definition.
'small volume about one pursuing girl' is the subsidiary indication.
'small volume' becomes 'cc' (cubic centimetre).
'about' means one lot of letters goes inside another.
'one' becomes 'i' (roman numeral).
'pursuing' says to put letters next to each other.
'girl' becomes 'lass' (lass is a kind of girl**).
'i' put after 'lass' is 'lassi'.
'cc' enclosing 'lassi' is 'classic'.
'is' is the link.
(Other definitions for classic that I've seen before include "Definitive; of recognised excellence", "The Derby or The Oaks, for example", "Recognised as excellent", "Remarkably typical", "Recognised as the best", "Time-honoured", "Great piece of creative writing", "Work of enduring excellance", "First-rate" and "Simple, neat and elegant".)