Attack in which sergeant-major captures rocky hill (5)
I believe the answer is:
storm
'attack' is the definition.
(storming is a kind of attacking)
'sergeant-major captures rocky hill' is the wordplay.
'sergeant major' becomes 'sm' (abbreviation for sergeant major).
'captures' means one lot of letters goes inside another.
'rocky hill' becomes 'tor' (I've seen this before).
'sm' enclosing 'tor' is 'STORM'.
'in which' acts as a link.
(Other definitions for storm that I've seen before include "- drain; - cloud" , "Trouble" , "Wind force 10 on Beaufort scale" , "Destructive wind" , "Atmospheric disturbance, assault (stronghold)," .)