Is it all the 5 session ID which is running SP A will start again with different plan or it's still executing using the same execution plan which previously it used before clear the cache
Clearing a cache, in your case to be precise procedure cache, will not affect already running queries. When you execute a query it is compiled and plans are generated and the most cost effective plan is selected and stored, post this query is run by sql server based on the plan. Clearing the cache removes the plan but will not affect already running queries. But, a new SP execution request will now first create new plan and then run it.