NCO securing hill in assault (5)
I believe the answer is:
storm
'assault' is the definition.
(storm is a kind of assault)
'nco securing hill' is the wordplay.
'nco' becomes 'sm' (abbreviation for sergeant major, a type of NCO).
'securing' means one lot of letters goes inside another.
'hill' becomes 'tor' (tor is a kind of hill).
'sm' going around 'tor' is 'STORM'.
'in' acts as a link.
(Other definitions for storm that I've seen before include "Sudden attack" , "- drain; - cloud" , "Squall" , "Force 10 blow" , "Very unfavourable climate" .)