The most common and most likely answer to this crossword clue is the 5 letter word SYLPH
I've seen this crossword clue in The Times
S
Y
L
P
H
More clues leading to the result SYLPH
Buddy explains!

The straight part
We know that "Pretty gir" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pretty gir = sylph.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SYLPH mean?
SYLPH noun- An invisible being of the air.
- The elemental being of air, usually female.
- (by extension) A slender woman or girl, usually graceful and sometimes with the implication of sublime station over everyday people.
- Any of the mainly dark green and blue hummingbirds (genus Aglaiocercus), the male of which has a long forked tail.