One regretted going topless after consuming dope (6)
I believe the answer is:
united
'one' is the definition.
(similar in meaning)
'regretted going topless after consuming dope' is the wordplay.
'regretted' becomes 'rued' ('rue' can be a synonym of 'regret').
'going topless' means to remove the first letter.
'after consuming' indicates putting letters inside.
'dope' becomes 'nit' (I've seen this in other clues).
'rued' with its first letter taken off is 'ued'.
'ued' enclosing 'nit' is 'UNITED'.
(Other definitions for united that I've seen before include "Untied (anag.)" , "Of one mind" , "In agreement" , "Joined - football club" , "Unified" .)