Pure niece finally in family group (5)
I believe the answer is:
clean
'pure' is the definition.
(similar in meaning)
'niece finally in family group' is the wordplay.
'finally' indicates one should take the final letters.
'in' is an insertion indicator.
'family group' becomes 'clan' (I've seen this before).
The final letter of 'niece' is 'e'.
'e' placed within 'clan' is 'CLEAN'.
(Other definitions for clean that I've seen before include "attentive to personal hygiene" , "far from blue" , "Unspoiled" , "Unsoiled" , "Not obscene; smooth" .)