Peculiar dictionary is unreliable - head for Chambers (13)
I believe the answer is:
idiosyncratic
'peculiar' is the definition.
(idiosyncratic means peculiar or particular)
'dictionary is unreliable head for chambers' is the wordplay.
'unreliable' is an anagram indicator.
'head for' says to take the initial letters.
The first letter of 'chambers' is 'c'.
'dictionary'+'is'='dictionaryis'
'dictionaryis' with letters rearranged gives 'idiosyncrati'.
'idiosyncrati'+'c'='IDIOSYNCRATIC'
(Other definitions for idiosyncratic that I've seen before include "Distinctively individual" , "Eccentric; dictionary (sic) (anag.)" , "Peculiar to the individual" , "Having a peculiar temperament" , "Distinctive, peculiar" .)