Fruit concealed by harlequin? Certainly (6)
I believe the answer is:
quince
'fruit' is the definition.
(quince is a kind of fruit)
'concealed by harlequin? certainly' is the wordplay.
'concealed' indicates a hidden word.
'QUINCE' can be found hidden inside 'by harlequin certainly'.
(Other definitions for quince that I've seen before include "Golden fruit" , "Shakespearean part?" , "Acid fruit used in jellies" , "Acid-flavoured fruit" , "Preserve material" .)