The one and only at the inside, segregate (7)
I believe the answer is:
isolate
'segregate' is the definition.
(thesaurus)
'the one and only at the inside' is the wordplay.
'the one' becomes 'i' (Roman numeral).
'and' says to put letters next to each other.
'only' becomes 'sole' ('sole' can be a synonym of 'only'**).
'the inside' indicates putting letters inside.
'sole' placed around 'at' is 'solate'.
'i'+'solate'='ISOLATE'
(Other definitions for isolate that I've seen before include "Point out" , "Put into quarantine" , "Eat soil (anag)" , "Segregate. close off" , "Maroon" .)