Keen enthusiast, guy CIA harassed (6)
I believe the answer is:
maniac
'keen enthusiast' is the definition.
'maniac' can be an answer for 'enthusiast' (I've seen this before). I'm not certain of the 'keen' bit.
'guy cia harassed' is the wordplay.
'guy' becomes 'man ' (guy can informally mean a man).
'harassed' indicates an anagram.
'cia' anagrammed gives 'iac'.
'man'+'iac'='MANIAC'
Can you help me to learn more?
(Other definitions for maniac that I've seen before include "Lunatic or person behaving wildly" , "Nutter" , "Insane person" , "Wild, perhaps violent, type" , "Very reckless person" .)