Unimportant child (5)
I believe the answer is:
minor
I believe this clue is a double definition.
'unimportant' is the first definition.
(I've seen this in another clue)
'child' is the second definition.
(I know that child can be written as minor)
(Other definitions for minor that I've seen before include "One hasn't reached legal majority" , "Of small importance" , "Under-age or of little importance" , "Less important" , "Morris car; tending to melancholy (mus.)" .)