Take into account, when you mull over (8)
I believe the answer is:
consider
'mull over' is the definition.
(I know that consider is a more specific form of the action mull over)
'take into' is the wordplay.
'take' becomes 'con' (I've seen this in another clue).
'into' becomes 'sider' (I am not sure about this - if you are sure you should believe this answer much more).
'con'+'sider'='CONSIDER'
'account when you' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for consider that I've seen before include "Go over something in one's mind" , "Think carefully about" , "Deliberate" , "Cogitate" , "Deem to be, believe" .)