Thought that's within: inside, always (4)
I believe the answer is:
'thought' is the definition.
(idea is a kind of thought)
'within inside always' is the wordplay.
'within' indicates a hidden word.
'IDEA' is hidden in the letters of 'inside always'.
'that's' is the link.
(Other definitions for idea that I've seen before include "a wheeze", "Purpose", "Notion, belief", "Flash of inspiration?", "It comes to mind".)