Be left secure in Ulster city (7)
I believe the answer is:
belfast
'ulster city' is the definition.
(I've seen this in another clue)
'be left secure' is the wordplay.
'left' becomes 'l' (common abbreviation).
'secure' becomes 'fast' (similar in meaning).
'be'+'l'+'fast'='BELFAST'
'in' is the link.
(Other definitions for belfast that I've seen before include "NI city" , "Northern Ireland home to Queen's University" , "Capital of Northern Ireland" , "Northern Ireland city" , "City on the Lagan" .)