American Indian conceals a singular wrinkle (6)
I believe the answer is:
crease
'wrinkle' is the definition.
('crease' can be a synonym of 'wrinkle')
'american indian conceals a singular' is the wordplay.
'american indian' becomes 'cree' (I've seen this before).
'conceals' indicates putting letters inside.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'a'+'s'='as'
'cree' placed around 'as' is 'CREASE'.
(Other definitions for crease that I've seen before include "Fold - wrinkle" , "ruck" , "Furrow, wrinkle" , "Furrow; cricket line" , "Folded line" .)