Such sorrow, if Reg is involved! (5)
I believe the answer is:
grief
'such sorrow' is the definition.
'grief' can be an answer for 'sorrow' (I've seen this before). I'm not sure about the 'such' bit.
'if reg is involved' is the wordplay.
'is involved' is an anagram indicator (involved can mean confusing or complex).
'if'+'reg'='ifreg'
'ifreg' with letters rearranged gives 'GRIEF'.
Can you help me to learn more?
(Other definitions for grief that I've seen before include "Great sorrow at someone's death" , "Intense sorrow caused by loss of a loved one" , "Great mourning" , "Misery" , "Suffering" .)