Sandhurst NCO involved in what’s not a search for military success (6,8)
I believe the answer is:
norman conquest
'military success' is the definition.
(military success for the Normans)
'sandhurst nco involved in what's not a search' is the wordplay.
'sandhurst' becomes 'RMA' (RMA Sandhurst, Royal Military Academy Sandhurst).
'involved in' indicates putting letters inside.
'what's not' becomes 'non' (eg non-verbal means what's not verbal).
'a search' becomes 'quest' (quest is a kind of search).
'rma'+'nco'='rmanco'
'rmanco' put into 'non' is 'normancon'.
'normancon'+'quest'='NORMAN CONQUEST'
'for' is the link.
(Other definitions for norman conquest that I've seen before include "cross-Channel triumph" , "immigration from France" , "Historic success for French" , "William's acquisition" .)