Existing as an original part of one Butlin redesigned (7)
I believe the answer is:
in-built
'existing as an original part' is the definition.
I can't tell whether this defines the answer.
'one butlin redesigned' is the wordplay.
'one' becomes 'i' (Roman numeral).
'redesigned' is an anagram indicator.
'butlin' is an anagram of 'nbuilt'.
'i'+'nbuilt'='IN-BUILT'
'of' is the link.
(Other definitions for in-built that I've seen before include "Existing as an essential constituent" , "Constructed as integrated part" , "Integral" , "Incorporated as part of structure" .)