Dancing in mask with new family member (7)
I believe the answer is:
kinsman
'family member' is the definition.
(I know that kinsman is a type of family member)
'dancing in mask with new' is the wordplay.
'dancing' indicates anagramming the letters.
'with' indicates putting letters inside.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'in'+'mask'='inmask'
'inmask' with letters rearranged gives 'kisman'.
'kisman' going around 'n' is 'KINSMAN'.
(Other definitions for kinsman that I've seen before include "Male relation" , "A blood relation" , "(Male) relative" , "One of the family" , "Family member" .)