Philosopher that's always right (4)
I believe the answer is:
ayer
'philosopher' is the definition.
(AJ Ayer)
'always right' is the wordplay.
'always' becomes 'aye' (archaic term for 'always').
'right' becomes 'r' (common abbreviation).
'aye'+'r'='AYER'
'that's' acts as a link.
(Other definitions for ayer that I've seen before include "2 [PHILOSOPHER]" , "A.J. - - , philosopher" , "A. J. -, English philosopher" .)