The most common and most likely answer to this crossword clue is the 7 letter word RUNDOWN
I've seen this crossword clue in The Daily Telegraph
R
U
N
D
O
W
N
Buddy explains!
The straight part
We know that "reduction in numbers" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reduction in numbers = rundown.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 RUNDOWN mean?
RUNDOWN adjective- (of a person) Tired and exhausted.
- (of a place) Decrepit.
- (of a clockwork mechanism) Having the spring unwound.
- (chiefly with definite article "the") A rough outline of a topic or situation.
- A defensive play in which the runner is caught between two fielders, who steadily converge to tag the runner out.
- A Caribbean stew of meat or fish (typically mackerel) with reduced coconut milk, yam, tomato, onion and seasonings.
- A reduction, e.g. of an activity, or in the size of something, such as a fleet.