Doll oddly found in fairground attraction is an enigma (6)
I believe the answer is:
riddle
'an enigma' is the definition.
('riddle' can be a synonym of 'enigma')
'doll oddly found in fairground attraction' is the wordplay.
'oddly' indicates alternate letters.
'found in' means one lot of letters goes inside another.
'fairground attraction' becomes 'ride'.
The alternating letters of 'doll' are 'dl'.
'dl' inserted inside 'ride' is 'RIDDLE'.
'is' is the link.
(Other definitions for riddle that I've seen before include "Kind of puzzle or coarse sieve" , "Make many openings" , "sort out" , "It may have an answer" , "Ingenious puzzle - sieve" .)