To understand why this is, it might be a good idea to review the basics of how a cryptic clue works.
Consider the following clue (taken from one of my own PSE questions):
School of thought puts pin in charged oxygen atom (7)
There are different parts to a cryptic clue. The two most prominent ones are the definition and the wordplay. In this case, the definition is:
school of thought
and the rest of the clue is the wordplay. The definition must be at the very beginning or the very end of the clue.
The wordplay can be parsed like so:
A charged atom is called an ion, and the atomic symbol for oxygen is o, giving us o ion.
The word "puts" is what's called an indicator, which tells us how to parse the clue. In this case, we should put the word "pin" inside of "o ion", giving us opinion, which is a synonym for "school of thought."
How this clue works:
One left a girl as a consequence (3)
The definition is
As a consequence, meaning end.
To get there, turn "a girl" into
Enid, a girl's name.
"one left" actually means
to take away a letter from Enid, giving us end, which is the solution.
Credit to @Stiv's answer on how to solve the clue; This answer goes more in-depth about how the clue actually works.