English radical taking overdose crumbled away (6)
I believe the answer is:
eroded
'crumbled away' is the definition.
(eroding is a kind of crumbling away)
'english radical taking overdose' is the wordplay.
'english' becomes 'e' (abbreviation).
'radical' becomes 'red' (I've seen this in other clues).
'taking' means one lot of letters goes inside another.
'overdose' becomes 'od' ('o.d.' can be a synonym of 'overdose').
'red' enclosing 'od' is 'roded'.
'e'+'roded'='ERODED'
(Other definitions for eroded that I've seen before include "Eaten away" , "Wore away" , "Worn away by time, wind or water" , "Worn away, ground down" , "Worn away by weather or water" .)