The most common and most likely answer to this crossword clue is the 3 letter word DOT
D
O
T
More clues leading to the result DOT
Buddy explains!

The straight part
We know that "Mark/little Dorothy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mark/little dorothy = dot.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 DOT mean?
DOT noun- A small, round spot.
- (grammar) A punctuation mark used to indicate the end of a sentence or an abbreviated part of a word; a full stop; a period.
- A point used as a diacritical mark above or below various letters of the Latin script, as in Ȧ, Ạ, Ḅ, Ḃ, Ċ.
- A symbol used for separating the fractional part of a decimal number from the whole part, for indicating multiplication or a scalar product, or for various other purposes.
- One of the two symbols used in Morse code.
- A lump or clot.
- Anything small and like a speck comparatively; a small portion or specimen.
- A dot ball.
- Buckshot, projectile from a "dotty" or shotgun
- To cover with small spots (of some liquid).
- To add a dot (the symbol) or dots to.
- To mark by means of dots or small spots.
- To mark or diversify with small detached objects.
- To punch (a person).
- Dot product of the previous vector and the following vector.
- A dowry.