Describing height, 'petite' could be one (7)
I believe the answer is:
epithet
'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'describing height petite could be' is the wordplay.
'describing' indicates putting letters inside (describe can mean to form the outline of).
'height' becomes 'h' (abbreviation in geometry).
'could be' indicates an anagram.
'petite' anagrammed gives 'epitet'.
'h' going inside 'epitet' is 'EPITHET'.
Can you help me to learn more?
(Other definitions for epithet that I've seen before include "Descriptive work or phrase" , "Word describing" , "qualifier" , "Phrase expressing some quality of a person or thing" , "nickname" .)