Part of White Mountains seen in detail (4)
I believe the answer is:
item
'detail' is the definition.
('item' can be a synonym of 'detail')
'part of white mountains seen' is the wordplay.
'part of' says the answer is hidden in the clue.
'ITEM' is hidden in the letters of 'white mountains seen'.
'in' acts as a link.
(Other definitions for item that I've seen before include "Report" , "Boyfriend and girlfriend" , "One thing - two people, perhaps" , "Detail (in a list)" , "Piece in paper" .)