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 "Quintessential", "Timeless-quality work; a race", "Having qualities like those of acknowledged art", "Serving as a standard - of the highest quality", "Work of enduring excellance", "Flat race", "Time-honoured", "Derby, say", "Simple, neat and elegant" and "Good example of its kind".)