A number of thieves (5)
I believe the answer is:
forty
'a number' is the definition.
(forty is a kind of number)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for forty that I've seen before include "number of penitential days" , "Kip" , "When life begins" , "-- winks, a short sleep" , "Two-score" .)