The most common and most likely answer to this crossword clue is the 4 plus 5 letter word SURE THING
S
U
R
E
T
H
I
N
G
More clues leading to the result SURE THING
- Their guns will go off, of course
- It's certain their guns will go off
- … a certain success? Certainly!
- Hunger — it’s terrible, no doubt
- Hungriest to change? Absolutely
- It is likely to succeed — rush, get in lively!
- Remove their guns, OK?
- It’s hunger possibly causing nap
- & 23a The eccentric covering web activity’s fine, of course
- Certainly, doctor hit nurse with end of ring
Buddy explains!

The straight part
We know that "nap" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nap = sure thing.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.