Fish about for a plan (4)
I believe the answer is:
idea
'for a plan' is the definition.
(both can mean an intention)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for idea that I've seen before include "Purpose" , "Notion, concept" , "Plan" , "Something imagined in the mind" , "intellectual product" .)