Wife has the plan to sharpen (4)
I believe the answer is:
'sharpen' is the definition.
(whetting is a kind of sharpening)
'wife has the plan' is the wordplay.
'wife' becomes 'w'.
'has' is a charade indicator (letters next to each other).
'plan' indicates anagramming the letters (plan can mean organise).
'the' anagrammed gives 'het'.
'to' is the link.
(Other definitions for whet that I've seen before include "grind" , "Sharpen by grinding" , "Sharpen - stimulate" , "Sharpen, appetite or razor" , "'Sharpen, appetite say (4)'" .)