Elaborate nest swallows case in slyness (6)
I believe the answer is:
dressy
'elaborate' is the definition.
(I've seen this before)
'nest swallows case in slyness' is the wordplay.
'nest' becomes 'drey' (I've seen this in other clues).
'swallows' is an insertion indicator.
'case in' means to remove the middle letters (the letters which encase the word are used).
'slyness' with its centre removed is 'ss'.
'drey' enclosing 'ss' is 'DRESSY'.
(Other definitions for dressy that I've seen before include "Stylish with regard to clothes" , "Sartorially elegant" , "(Of clothes) suitable for a smart occasion" , "Wearing stylish clothing" , "Smartly clothed" .)