Inspiration from what's concealed by seaside alders (4)
I believe the answer is:
Here is my best explanation:
'inspiration' is the definition.
'what's concealed by seaside alders' is the subsidiary indication.
'what's concealed' says the answer is hidden in the clue.
'idea' is hidden within 'by seaside alders'.
'from' is the link.
(Other definitions for idea that I've seen before include "Purpose", "Sudden inspiration?", "'Notion, thought (4)'", "Brainwave", "Plan", "Something imagined in the mind", "A brainwave?", "Notion, image", "Theory" and "A thought or notion".)