Complex sentence: "But for" or actual causation can be difficult to prove and usually results in a "trial-within-a-trial" which delves into the facts of the case for which the client originally retained the attorney.
Split into simpler sentences: But for or actual causation can be difficult to prove. If the malpractice alleged occurred in litigation, the legal malpractice case may result in a "trial-within-a-trial" which delves into the facts of the case for which the client originally retained the attorney.