Tailor designed round bowler, originally circular (7)
I believe the answer is:
orbital
'circular' is the definition.
(thesaurus)
'tailor designed round bowler originally' is the wordplay.
'designed' indicates an anagram (designed with the letters).
'round' means one lot of letters goes inside another.
'originally' indicates taking the first letters.
The first letter of 'bowler' is 'b'.
'tailor' anagrammed gives 'orital'.
'orital' going around 'b' is 'ORBITAL'.
(Other definitions for orbital that I've seen before include "going round the city" , "Like a sort of road" , "Going round - the M25?" , "(Of a road) going around a town" , "Like a ring road" .)