Heartless son longing to be a telltale (6)
I believe the answer is:
snitch
'a telltale' is the definition.
(thesaurus)
'heartless son longing' is the wordplay.
'heartless' suggests removing the centre.
'longing' becomes 'itch' (I've seen this before).
'son' with its centre removed is 'sn'.
'sn'+'itch'='SNITCH'
'to be' acts as a link.
(Other definitions for snitch that I've seen before include "Telltale (sl.)" , "Grass" , "Steal; inform (on someone)" , "Inform on someone in slangy way" , "who tells on his classsmate?" .)