Read out two letters too many (6)
I believe the answer is:
excess
'read out' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'two letters too many' is the wordplay.
'two letters' means two replacements for 'letter'.
'letter' becomes 'ex' (X).
'letter' becomes 'c'.
'too many' becomes 'ess' (I can't justify this - if you can you should give a lot more credence to this answer).
'ex'+'c'+'ess'='EXCESS'
Can you help me to learn more?
(Other definitions for excess that I've seen before include "Too much - going too far" , "Over and above what's needed" , "Outrageous behaviour; extra payment" , "Intemperance" , "More than is needed, desired, or required" .)