Scared of American fellow mounting attack (6)
I believe the answer is:
afraid
'scared' is the definition.
('afraid' can be similar in meaning to 'scared')
'american fellow mounting attack' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'fellow' becomes 'f' (abbreviation - of a society etc.).
'mounting' says to put letters next to each other.
'attack' becomes 'raid' (raiding is a kind of attacking).
'a'+'f'+'raid'='AFRAID'
'of' is the link.
(Other definitions for afraid that I've seen before include "Fearful, anxious" , "cowardly" , "Filled with concern" , "wind up" , "Timorous" .)