Intense nervous excitement always shown by fathead (5)
I believe the answer is:
fever
'intense nervous excitement' is the definition.
'fever' can be an answer for 'excitement' (fever is a kind of excitement). I'm not certain of the 'intense nervous' bit.
'always shown by fathead' is the wordplay.
'always' becomes 'ever' (synonyms).
'shown by' is a charade indicator (letters next to each other).
'fathead' becomes 'f' (head letter of 'fat').
'ever' after 'f' is 'FEVER'.
Can you help me to learn more?
(Other definitions for fever that I've seen before include "Reason for temperature" , "Elvis hit" , "eg Malaria" , "Nervous excitement, high temperature" , "State of heightened excitement" .)