Experimentation is an ordeal, also a mistake (5,3,5)
I believe the answer is:
trial and error
'experimentation' is the definition.
(trial and error is a kind of experimentation)
'an ordeal also a mistake' is the wordplay.
'an ordeal' becomes 'trial' (I've seen this in another clue).
'also' becomes 'and' (both can mean in addition).
'a mistake' becomes 'error' ('error' can be a synonym of 'mistake').
'trial'+'and'+'error'='TRIAL-AND-ERROR'
'is' is the link.
(Other definitions for trial and error that I've seen before include "testing process" , "Method using practical experience rather than theory" , "Experiment" , "Random experimentation" , "Problem solving by experience" .)