Flower in hair isn't (4)
I believe the answer is:
iris
'flower' is the definition.
(I've seen this before)
'in hair isn't' is the wordplay.
'in' says the answer is hidden in the clue.
'IRIS' is hidden in the letters of 'hair isnt'.
(Other definitions for iris that I've seen before include "Garden flower - part of eye" , "Bright flower or eye part" , "Eye diaphragm" , "Flag - part of eye" , "Flowers in the eye?" .)