Night SAS lost battle (8)
I believe the answer is:
'battle' is the definition.
(I know that Hastings is a pitched battle)
'night sas lost' is the wordplay.
'lost' is an anagram indicator.
'nightsas' is an anagram of 'HASTINGS'.
(Other definitions for hastings that I've seen before include "His angst (anag.)" , "somewhere in Sussex" , "South coast resort" , "Ten Sixty-Six battle" , "Harold's objective?" .)