In truth, lesson's getting hard (8)
I believe the answer is:
ruthless
'hard' is the definition.
(thesaurus)
'in truth lesson's getting' is the wordplay.
'in' says the answer is hidden in the clue.
'RUTHLESS' can be found hidden inside 'truth lessons getting'.
(Other definitions for ruthless that I've seen before include "Hustlers can be without mercy" , "Hard-hearted" , "Lacking pity or mercy" , "Unsparing" , "Without pity, merciless" .)