How to use GROUP BY on a CLOB Column

i'm trying to use this QUERY which contains a Clob Column (flow.IDFONCTIONNEL)

SELECT  
flow.flowid,   
min(flow.CONTEXTTIMESTAMP) contextTime,   
flow.STATUT,
flow.IDFONCTIONNEL,
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, flow.IDFONCTIONNEL
ORDER BY contextTime desc

When I run this query, I get the error ORA-00932: inconsistent data types Expected got CLOB This is because the column flow.IDFONCTIONNEL is a CLOB data type. If I comment this column from select clause it works fine but I need this column in the output. I have seen a post telling to try to use DBMS_LOB.SUBSTR to try avoid this problem, so i tryed :

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, idf1 ,idf2 
ORDER BY contextTime desc

But i get ORA-00904: "IDF2" invalid identifier. Anyone as an idea of why it is not working ? Thanks a lot