Stop account, nearly all losing millions (6)
I believe the answer is:
accost
'stop' is the definition.
(I've seen this before)
'account nearly all losing millions' is the wordplay.
'account' becomes 'acc'.
'nearly all' becomes 'most' (I've seen this in another clue).
'losing' indicates named letters should be taken away.
'millions' becomes 'm' (abbreviation).
'most' with 'm' taken away is 'ost'.
'acc'+'ost'='ACCOST'
(Other definitions for accost that I've seen before include "Make bold approach to" , "Challenge" , "Stop and pester" , "Approach and address boldly" , "Approach boldly or aggressively" .)