Too much is done badly without looking, at heart (8)
I believe the answer is:
overkill
'too much' is the definition.
('overkill' can mean excessive or too much)
'done badly without looking at heart' is the wordplay.
'done' becomes 'over' (I've seen this before).
'badly' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
'without looking' becomes 'k' (I can't justify this - if you can you should give a lot more credence to this answer).
'at heart' indicates putting letters inside.
'over'+'ill'='overill'
'overill' going around 'k' is 'OVERKILL'.
'is' acts as a link.
(Other definitions for overkill that I've seen before include "Ridiculous excess" , "Surplus of capacity for task - sounds lethal" , "Action far more drastic than necessary" , "Excessive effort for task - sounds deadly" , "Excessive action, armament" .)