The most common and most likely answer to this crossword clue is the 8 letter word REACTIVE
I've seen this crossword clue in The Guardian
R
E
A
C
T
I
V
E
More clues leading to the result REACTIVE
Buddy explains!

The straight part
We know that "Likely to respond" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case likely to respond = reactive.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 REACTIVE mean?
REACTIVE adjective- That reacts or responds to a stimulus
- That readily takes part in reactions
- Characterized by induction or capacitance rather than resistance.
- Reacting to the past rather than anticipating the future, not predictive.