Mature people from America strangely lust after diamonds (6)
I believe the answer is:
adults
'mature people' is the definition.
(I've seen this before)
'america strangely lust after diamonds' is the wordplay.
'america' becomes 'a' (common abbreviation - e.g. in organisation names).
'strangely' is an anagram indicator.
'after' means one lot of letters go next to another.
'diamonds' becomes 'd' (abbreviation in bridge say).
'lust' is an anagram of 'ults'.
'ults' put after 'd' is 'dults'.
'a'+'dults'='ADULTS'
'from' acts as a link.
(Other definitions for adults that I've seen before include "Fully grown animals" , "Mature persons" , "Mature beings" , "Mature or fully-grown people" , "Mature creatures" .)