It would be simple to put one in the hub (5)
I believe the answer is:
naive
'it would be simple to' is the definition.
The definition suggests a singular noun but the answer is not.
'one in the hub' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in the' indicates putting letters inside.
'hub' becomes 'nave' (I've seen this in another clue).
'i' going into 'nave' is 'NAIVE'.
'put' is the link.
Can you help me to learn more?
(Other definitions for naive that I've seen before include "Simple in outlook" , "Rather too simple and ingenuous" , "Rather too simple and gullible" , "Credulous, unworldly" , "Callow" .)