Fearful that oaf, losing head, might attack (6)
I believe the answer is:
afraid
'fearful' is the definition.
(associated in meaning)
'oaf losing head might attack' is the wordplay.
'losing head' means to remove the first letter.
'might attack' becomes 'raid' (I am not sure about this - if you are sure you should believe this answer much more).
'oaf' with its initial letter taken off is 'af'.
'af'+'raid'='AFRAID'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for afraid that I've seen before include "Not confident" , "Regretfully" , "Scared, apprehensive" , "Feeling anxiety or fear" , "feeling scared" .)