One playing with toys, liar (8)
I believe the answer is:
solitary
'one' is the definition.
(a solitary thing can mean only one thing)
'playing with toys liar' is the wordplay.
'playing' indicates an anagram.
'with' says to put letters next to each other.
'toys'+'liar' is 'toysliar'.
'toysliar' is an anagram of 'SOLITARY'.
(Other definitions for solitary that I've seen before include "'Sole, lone (8)'" , "Lacking companions" , "friendless" , "Left alone" , "Lone; isolated" .)