The most common and most likely answer to this crossword clue is the 3 letter word BUM
I've seen this crossword clue in The Times
B
U
M
More clues leading to the result BUM
Buddy explains!

The straight part
We know that "summary dismissal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case summary dismissal = bum.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 BUM mean?
BUM noun- The buttocks.
- The anus.
- To sodomize; to engage in anal sex.
- An expression of annoyance.
- (sometimes derogatory) A homeless person, usually a man.
- A lazy, incompetent, or annoying person, usually a man.
- A player or racer who often performs poorly.
- A drinking spree.
- To ask someone to give one (something) for free; to beg for something.
- To stay idle and unproductive, like a hobo or vagabond; to loiter.
- To wet the end of a marijuana cigarette (spliff).
- Of poor quality or highly undesirable.
- Unfair.
- Injured and without the possibility of full repair, defective.
- Unpleasant or unhappy.
- To depress; to make unhappy.
- A humming noise.
- To make a murmuring or humming sound.
- A bumbailiff.