Concentrating hard on, aim (6)
I believe the answer is:
intent
This clue is a triple definition!
'concentrating' is the first definition.
(I've seen this before)
'hard' is the second definition.
(similar in meaning)
'on' acts as a link.
'aim' is the third definition.
(I know that aim can be written as intent)
(Other definitions for intent that I've seen before include "Purpose, design" , "Design, aim" , "Objective" , "Eager" , "Bent on, determined to" .)