Rare element of maturity adult leaves out (7)
I believe the answer is:
yttrium
'rare element' is the definition.
(I've seen this before)
'maturity adult leaves out' is the wordplay.
'adult' becomes 'a' (abbreviation).
'leaves' indicates named letters should be taken away.
'out' indicates an anagram (out can mean wrong or inaccurate).
'maturity' with 'a' taken out is 'mturity'.
'mturity' with letters rearranged gives 'YTTRIUM'.
'of' acts as a link.
(Other definitions for yttrium that I've seen before include "Silvery metallic element" , "Metallic element, symbol Y" , "Rare earth element" , "12 [ELEMENT]" , "Chemical element; um, try it (anag.)" .)