It's essential for seeing part of minaret in Alhambra (6)
I believe the answer is:
retina
'it's essential for seeing' is the definition.
(part of the eye)
'part of minaret in alhambra' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'RETINA' can be found hidden inside 'minaret in alhambra'.
(Other definitions for retina that I've seen before include "Ocular membrane" , "Back surface of eye" , "What's behind the humour" , "Bad tear in part of the eye" , "It helps one to see" .)