A bit of a cat (4)

(Source: London Evening Standard)

General meaning ⇒ "a bit of" ⇒ a word for a basic piece of something

"cat" ⇒ another word for cat? ⇒ e.g. a male cat ⇒ "tom"

"a cat" ⇒ consider as "a" plus the above "cat"

Answer ⇒ atom