Fools taken aback by heartless guy being mean (6)
I believe the answer is:
stingy
'being mean' is the definition.
('stingy' can be similar in meaning to 'mean')
'fools taken aback by heartless guy' is the wordplay.
'fools' becomes 'nits' (I've seen this in other clues**).
'taken aback' shows that the letters should be reversed in order.
'by' is a charade indicator (letters next to each other).
'heartless' says to hollow out the word (remove centre letters).
'guy' with its middle taken out is 'gy'.
'nits' back-to-front is 'stin'.
'stin'+'gy'='STINGY'
(Other definitions for stingy that I've seen before include "reluctant to spend money" , "Mean, parsimonious - ouch?" , "Tight-fisted, like 25 and 22 Across" , "Wasps are" , "near" .)