Challenge to serve up sample of horseradish (4)
I believe the answer is:
dare
'challenge' is the definition.
(daring is a kind of challenging)
'serve up sample of horseradish' is the wordplay.
'serve up' is a reversal indicator.
'sample of' indicates the answer is hidden within the clue (I've seen this in other clues).
'erad' is hidden in the letters of 'horseradish'.
'erad' reversed gives 'DARE'.
'to' is the link.
(Other definitions for dare that I've seen before include "Throw down the gauntlet" , "Be bold enough (to)" , "Venture to read" , "Challenge of courage" , "Be courageous enough to try or do" .)