Two letters read out, that's more than enough (6)
I believe the answer is:
excess
'more than enough' is the definition.
(I've seen this before)
'two letters read out' is the wordplay.
'two letters' means two substitutions for 'letter'.
'letter' becomes 'ex' (X).
'letter' becomes 'c'.
'read out' becomes 'ess' (I am not sure about this - if you are sure you should believe this answer much more).
'ex'+'c'+'ess'='EXCESS'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for excess that I've seen before include "Over and above what's needed" , "Too much, surplus" , "Too much - going too far" , "Amount that is more than is needed" , "Glut, superfluity" .)