I remain upset about grand headache (8)
I believe the answer is:
migraine
'headache' is the definition.
(migraine is a kind of headache)
'i remain upset about grand' is the wordplay.
'remain upset' becomes 'graine' (I can't explain this - if you can you should believe this answer much more).
'about' means one lot of letters go next to another.
'grand' becomes 'm' ('M' can be a synonym of 'grand').
'i'+'graine'='igraine'
'igraine' put after 'm' is 'MIGRAINE'.
Can you help me to learn more?
(Other definitions for migraine that I've seen before include "Severe headache often accompanied by nausea" , "It brings severe headache" , "terrible pain" , "From mare gin I get a severe headache" , "problem for the head" .)