Discovered tragic king not losing heart (6)
I believe the answer is:
learnt
'discovered' is the definition.
(I've seen this in another clue)
'tragic king not losing heart' is the wordplay.
'tragic king' becomes 'Lear' (Shakespeare's King Lear).
'losing heart' suggests removing the centre.
'not' with its middle removed is 'nt'.
'lear'+'nt'='LEARNT'
(Other definitions for learnt that I've seen before include "Heard" , "Found out" , "Aquired knowledge or skill" , "Picked up" , "Gathered" .)