Unreliable scoundrel penetrating heart of American (7)
I believe the answer is:
erratic
'unreliable' is the definition.
('erratic' can be similar in meaning to 'unreliable')
'scoundrel penetrating heart of american' is the wordplay.
'scoundrel' becomes 'rat' (I've seen this in other clues).
'penetrating' is an insertion indicator.
'heart of' says to take the centre.
The middle of 'american' is 'eric'.
'rat' put into 'eric' is 'ERRATIC'.
(Other definitions for erratic that I've seen before include "Irregular, not dependable" , "Fickle" , "Irregular or uneven in movement or habit" , "Wild" , "Irregular - eccentric" .)