Part of helicopter shelled from hill (5)
I believe the answer is:
rotor
'part of helicopter' is the definition.
(a helicopter has a rotor)
'shelled from hill' is the wordplay.
'shelled' indicates the central letters (the outer letters or shell are removed).
'hill' becomes 'tor' (tor is a kind of hill).
The centre of 'from' is 'ro'.
'ro'+'tor'='ROTOR'
(Other definitions for rotor that I've seen before include "Distributor arm" , "Revolving dynamic part" , "Revolving mechanism" , "Reversible piece of moving machinery?" , "Turning part of a mechanism" .)