Pretty heartless getting thin (4)
I believe the answer is:
bony
'thin' is the definition.
(similar in meaning)
'pretty heartless' is the wordplay.
'pretty' becomes 'bonny' (I've seen this before).
'heartless' suggests removing the centre.
'bonny' with its middle taken out is 'BONY'.
'getting' acts as a link.
(Other definitions for bony that I've seen before include "Scrawny" , "Osseous" , "Skeletal" , "Angular" , "With skeleton showing through" .)