Able to be done (8)
(Other definitions for possible that I've seen before include "Capable of being achieved" , "Capable of happening" , "Conceivable, practicable" , "Feasible achievable" , "Achievable, able to be done" .)
'able to be done' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for feasible that I've seen before include "is likely to work" , "Base life (anag.)" , "workable" , "Capable of being done" , "An idea" .)