US writer, poet with change of heart, to get engrossed (8)
I believe the answer is:
bradbury
'us writer' is the definition.
(author Ray Bradbury)
'poet with change of heart to get engrossed' is the wordplay.
'poet with' becomes 'bra' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'change' becomes 'ado' (ado is a kind of change).
'of heart' means to look at the middle letters (I've seen 'heart of' mean this).
'to get engrossed' becomes 'bury' (as in burying or engrossing oneself in something).
The centre of 'ado' is 'd'.
'bra'+'d'+'bury'='BRADBURY'
(Another definition for bradbury that I've seen is " Ray -, Fahrenheit Four Five One author".)