Health food runs into prohibition (4)
I believe the answer is:
bran
'health food' is the definition.
(I've seen this in another clue)
'runs into prohibition' is the wordplay.
'runs' becomes 'r' (abbreviation in cricket).
'into' means one lot of letters goes inside another.
'prohibition' becomes 'ban' (ban is a kind of prohibition).
'r' placed within 'ban' is 'BRAN'.
(Other definitions for bran that I've seen before include "Horse-food in barn" , "Husks of corn" , "Horse food stored in barn" , "Type of edible fibre" , "Separated grain husks" .)