Small group of stars, reduced in total number (6)
I believe the answer is:
scutum
'small group of stars reduced in total number' is the definition.
I can't tell whether this defines the answer.
'reduced in total number' is the wordplay.
'reduced' becomes 'cut' (similar in meaning).
'in' means one lot of letters goes inside another.
'total number' becomes 'sum' (summing is a kind of numbering. I am not sure about the 'total' bit.).
'cut' inserted into 'sum' is 'SCUTUM'.
Can you help me to learn more?
(Other definitions for scutum that I've seen before include "lot of stars required" , "Shield" .)