Round button originally secured by tailor that's loose (7)
I believe the answer is:
orbital
'round' is the definition.
(thesaurus)
'button originally secured by tailor that's loose' is the wordplay.
'originally' suggests taking the first letters.
'secured by' is an insertion indicator (I've seen this in other clues).
'that's loose' indicates anagramming the letters (letters are loosened from their original positions).
The initial letter of 'button' is 'b'.
'tailor' is an anagram of 'orital'.
'b' going inside 'orital' is 'ORBITAL'.
(Other definitions for orbital that I've seen before include "Following a path around a heavenly body" , "(Of a road) going round a town" , "M25, say" , "Descriptive of the M25" , "Rob tail (anag.)" .)