Centre set up in Little Rock (4)
I believe the answer is:
core
'centre' is the definition.
(core is a kind of centre)
'set up in little rock' is the wordplay.
'set up' says the letters should be written in reverse.
'in' indicates a hidden word.
'eroc' is hidden in the letters of 'little rock'.
'eroc' back-to-front is 'CORE'.
(Other definitions for core that I've seen before include "Heal" , "Heart of the matter" , "Crux" , "The centre of an object" , "Central or most important part of something" .)