Key test capturing singular way to keep you looking thin (8)
I believe the answer is:
corsetry
'way to keep you looking thin' is the definition.
I can't judge whether this definition defines the answer.
'key test capturing singular' is the wordplay.
'key' becomes 'core' (both can mean majorly important).
'test' becomes 'try' (synonyms).
'capturing' indicates putting letters inside.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'core'+'try'='coretry'
'coretry' going around 's' is 'CORSETRY'.
Can you help me to learn more?
(Other definitions for corsetry that I've seen before include "that gives me a shapely figure!" , "stays involved in this" .)