Neglected sector on hem of zoot suit? (3,4)

'neglected sector' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe you can see an association between them that I can't see?

'on hem of zoot suit?' is the wordplay.
'on' says to put letters next to each other.
'hem of' suggests the final letters (hem can mean an edge or border).
'suit?' becomes 'hearts' (hearts is a playing card suit).
The final letter of 'zoot' is 't'.
't' put next to 'hearts' is 'THE ARTS'.

Can you help me to learn more?