Clue in truth interesting (4)
I believe the answer is:
hint
'clue' is the definition.
(synonyms)
'in truth interesting' is the wordplay.
'in' indicates a hidden word.
'HINT' is hidden within 'truth interesting'.
(Other definitions for hint that I've seen before include "Indirect clue" , "Tip-off" , "Small trace; clue" , "Thin sort of slight indication" , "'Slight indication, indirect suggestion (4)'" .)