Flower part pales unexpectedly (5)
I believe the answer is:
sepal
'flower part' is the definition.
(I've seen this before)
'pales unexpectedly' is the wordplay.
'unexpectedly' indicates anagramming the letters.
'pales' anagrammed gives 'SEPAL'.
(Other definitions for sepal that I've seen before include "Part of a flowering plant" , "Green part that forms a flower's calyx" , "bit of a bloomer" , "Part of a flower casing" , "flowery bit" .)