Guessing game leads to liars being devious (5,2,5)
I believe the answer is:
heads or tails
'guessing game' is the definition.
(game of predicting the result of a coin toss)
'leads to liars being devious' is the wordplay.
'leads' becomes 'heads' ('head' can be a synonym of 'lead').
'being devious' is an anagram indicator.
'to'+'liars'='toliars'
'toliars' anagrammed gives 'ortails'.
'heads'+'ortails'='HEADS OR TAILS'
(Other definitions for heads or tails that I've seen before include "Coin-tossing question" , "Choice" , "phrase?" .)