Artist taking very very long time effects destructive action (7)
I believe the answer is:
ravages
'effects destructive action' is the definition.
(ravage can mean to destroy)
'artist taking very very long time' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'taking' says to put letters next to each other.
'very' becomes 'v' (abbreviation).
'very long time' becomes 'ages' (ages can mean a very long period).
'ra'+'v'+'ages'='RAVAGES'
(Other definitions for ravages that I've seen before include "Causes great damage" , "a very bad series of effects" , "Sacks" .)