Exultant cry on entering eastern UK area (6)
I believe the answer is:
eureka
'exultant cry' is the definition.
(I've seen this before)
'on entering eastern uk area' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'entering' indicates putting letters inside.
'eastern' becomes 'e'.
'area' becomes 'a' (maths abbreviation).
'e'+'uk'='euk'
're' put inside 'euk' is 'eurek'.
'eurek'+'a'='EUREKA'
(Other definitions for eureka that I've seen before include "discovery that's brilliant" , "Archimedes's cry of discovery" , "Exclamation on discovering something" , "Cry announcing a brilliant discovery" , "alloy" .)