'Number Ten' and their constituents (8)
I believe the answer is:
thirteen
'number' is the definition.
(thirteen is a kind of number)
'ten and their constituents' is the wordplay.
'and' means one lot of letters go next to another (I've seen this in other clues).
'constituents' indicates anagramming the letters.
'ten' put after 'their' is 'theirten'.
'theirten' is an anagram of 'THIRTEEN'.
(Other definitions for thirteen that I've seen before include "Supper attendees" , "Cardinal" , "ill-omened figure" , "Number considered unlucky" , "this could be unlucky" .)