Possible Clues:

• I'd worn a buckle briefly
• How antidisestablishmentarianism might be summarised concisely
• Where letters are usually found to be concise
• How the eight longest Across answers are clued
• Succinctly (3 words)
• To sum up, one tree, falling over, died
• Police officer hugging tree sent up briefly
• To summarize Origin of Species leaves Darwin so excited
• Not at length
• I go under, making retraction about a summing up