The most common and most likely answer to this crossword clue is the 8 letter word DWINDLES
I've seen this crossword clue in Financial Times
D
W
I
N
D
L
E
S
Buddy explains!
The straight part
We know that "is getting shorter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case is getting shorter = dwindles.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 DWINDLES mean?
DWINDLES verb- To decrease, shrink, diminish, reduce in size or intensity.
- To fall away in quality; degenerate, sink.
- To lessen; to bring low.
- To break up or disperse.
- Various health problems and frailty observed in elderly people.