Some children always concerned with part of body (5)
I believe the answer is:
renal
'concerned with part of body' is the definition.
(relating to the kidneys)
'some children always' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'RENAL' is hidden within 'children always'.
(Other definitions for renal that I've seen before include "Concerning the kidneys" , "Referring to organ" , "around the kidney" , "Relating to kidneys" , "Concerned with organ" .)