Seaside resort has to keep smart (8)
I believe the answer is:
hastings
'seaside resort' is the definition.
(Hastings is an English seaside resort)
'has to keep smart' is the wordplay.
'to keep' indicates putting letters inside.
'smart' becomes 'sting' (both can mean to burn or hurt).
'has' enclosing 'sting' is 'HASTINGS'.
(Other definitions for hastings that I've seen before include "South coast resort" , "Sussex battle" , "Ten Sixty-Six battle" , "somewhere in Sussex" , "His angst (anag.)" .)