Ten moving into flat — five in this group (6)
I believe the answer is:
pentad
'five in this group' is the definition.
(a pentad is a group of five)
'ten moving into flat' is the wordplay.
'moving' indicates an anagram.
'into' indicates putting letters inside.
'flat' becomes 'pad' (person's home).
'ten' anagrammed gives 'ent'.
'ent' inserted inside 'pad' is 'PENTAD'.
(Other definitions for pentad that I've seen before include "Group of five" , "Enid Blyton's was famous" , "five years" , "Half a decade" , "small group" .)