Seems worried about one with a cat (7)
I believe the answer is:
siamese
'cat' is the definition.
(Siamese is an example)
'seems worried about one with a' is the wordplay.
'worried' indicates an anagram.
'about' indicates putting letters inside.
'one' becomes 'i' (Roman numeral**).
'with' is an insertion indicator.
'seems' with letters rearranged gives 'smese'.
'smese' placed around 'i' is 'simese'.
'simese' enclosing 'a' is 'SIAMESE'.
(Other definitions for siamese that I've seen before include "sort of twins" , "Cat - Thai?" , "cats?" , "Cat with slanting blue eyes and pale fur with darker points" , "like a cat" .)