How to use GROUP BY on a CLOB Column
GROUP BY can not use aliases defined in the same query block.
You can use this:
SELECT  
flow.flowid,   
min(flow.CONTEXTTIMESTAMP) contextTime,   
flow.STATUT,
DBMS_LOB.SUBSTR(flow.IDFONCTIONNEL,4000,1) as idf1,
DBMS_LOB.SUBSTR(flow.IDFONCTIONNEL,8000,4001) as idf2,
flow.ETAT
FROM Flux flow 
WHERE  flow.FLOWCODE = 'HELLO' 
AND flow.CONTEXTTIMESTAMP BETWEEN '06/01/20 11:36:21,566000000' AND '06/07/20 11:36:21,566000000' 
GROUP BY flow.flowid, flow.STATUT,flow.ETAT, DBMS_LOB.SUBSTR(flow.IDFONCTIONNEL,4000,1),
DBMS_LOB.SUBSTR(flow.IDFONCTIONNEL,8000,4001) 
ORDER BY contextTime desc 

Or something like this:
SELECT
flowid,
min(CONTEXTTIMESTAMP) contextTime,
STATUT,
idf1,
idf2,
ETAT
from
(
SELECT  
flow.flowid,   
flow.CONTEXTTIMESTAMP,   
flow.STATUT,
DBMS_LOB.SUBSTR(flow.IDFONCTIONNEL,4000,1) as idf1,
DBMS_LOB.SUBSTR(flow.IDFONCTIONNEL,8000,4001) as idf2,
flow.ETAT
FROM Flux flow 
WHERE  flow.FLOWCODE = 'HELLO' 
AND flow.CONTEXTTIMESTAMP BETWEEN '06/01/20 11:36:21,566000000' AND '06/07/20 11:36:21,566000000'
)
GROUP BY flowid, STATUT,ETAT, idf1 ,idf2 
ORDER BY contextTime desc