Regrets doctors taking pot (6)
I believe the answer is:
mourns
'regrets' is the definition.
(mourn can mean to regret something)
'doctors taking pot' is the wordplay.
'doctors' becomes 'MOS' (an MO is a Medical Officer).
'taking' is an insertion indicator.
'pot' becomes 'urn' (urn is a kind of pot).
'mos' going around 'urn' is 'MOURNS'.
(Other definitions for mourns that I've seen before include "Grieves for, feels sorrow for dead" , "Feels loss" , "Laments the loss of" .)