The most common and most likely answer to this crossword clue is the 5 letter word DECAY
I've seen this crossword clue in The Guardian
D
E
C
A
Y
Buddy explains!
The straight part
We know that "don’t last" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case don’t last = decay.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 DECAY mean?
DECAY noun- The process or result of being gradually decomposed.
- A deterioration of condition; loss of status or fortune.
- To deteriorate, to get worse, to lose strength or health, to decline in quality.
- (of organic material) To rot, to go bad.
- (of an unstable atom) To change by undergoing fission, by emitting radiation, or by capturing or losing one or more electrons.
- (of a quantum system) To undergo optical decay, that is, to relax to a less excited state, usually by emitting a photon or phonon.
- Loss of airspeed due to drag.
- To cause to rot or deteriorate.