Britain above changing short form (12)
I believe the answer is:
abbreviation
'short form' is the definition.
(abbreviation is a kind of form; 'short' suggests a short form)
'britain above changing' is the wordplay.
'changing' is an anagram indicator.
'britain'+'above'='britainabove'
'britainabove' is an anagram of 'ABBREVIATION'.
(Other definitions for abbreviation that I've seen before include "Device linking 1d, 7, 15 and 19" , "plane, for example?" , "Shortened form of a word or phrase" , "UFO, for example" , "BA, for example" .)