One hiding in front building disappeared (8)
I believe the answer is:
vanished
'disappeared' is the definition.
(vanishing is a kind of disappearing)
'one hiding in front building' is the wordplay.
'one' becomes 'i' (Roman numeral).
'hiding in' is an insertion indicator.
'front' becomes 'van' (I've seen this before).
'building' becomes 'shed' (shed is a kind of building**).
'van'+'shed'='vanshed'
'i' placed within 'vanshed' is 'VANISHED'.
(Other definitions for vanished that I've seen before include "disappeared from sight" , "disappeared from view" , "gone" , "Disappeared without trace" .)