Old, unchanged rock group (5)
I believe the answer is:
oasis
'rock group' is the definition.
(British band)
'old unchanged' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'unchanged' becomes 'asis' (I've seen this before).
'o'+'asis'='OASIS'
(Other definitions for oasis that I've seen before include "Place of peace" , "can provide welcome relief" , "Fertile patch in a desert" , "flower arranging stuff" , "Place of refreshment" .)