Crown originally in iron is adapted for tooth (7)
I believe the answer is:
incisor
'for tooth' is the definition.
(incisor is a kind of tooth)
'crown originally in iron is adapted' is the wordplay.
'originally' suggests taking the first letters.
'in' means one lot of letters goes inside another.
'adapted' is an anagram indicator.
The first letter of 'crown' is 'c'.
'iron'+'is'='ironis'
'ironis' with letters rearranged gives 'inisor'.
'c' inserted into 'inisor' is 'INCISOR'.
(Other definitions for incisor that I've seen before include "oral equipment" , "cutter" , "One takes a bite" , "Canine" , "something with bite" .)