Liar posted odd letters in error (5)
I believe the answer is:
lapse
'letters in error' is the definition.
'lapse' can be an answer for 'error' (lapse is a kind of error). I'm not certain of the 'letters in' bit.
'liar posted odd' is the wordplay.
'odd' means one should take alternating letters.
'liar'+'posted'='liarposted'
The alternate letters of 'liarposted' are 'LAPSE'.
Can you help me to learn more?
(Other definitions for lapse that I've seen before include "Brief slip; fall from faith" , "A slight error or a passage of time" , "Pales into omission" , "Error; expire" , "moral shortcoming" .)