Regressive ayatollah still restricts this foodstuff (7)
I believe the answer is:
shallot
'this foodstuff' is the definition.
(shallot is a kind of foodstuff)
'regressive ayatollah still restricts' is the wordplay.
'regressive' shows that the letters should be reversed in order (to regress is to go backwards).
'restricts' indicates a hidden word (the answer is held or restricted by the clue).
'tollahs' can be found hidden inside 'ayatollah still'.
'tollahs' back-to-front is 'SHALLOT'.
(Other definitions for shallot that I've seen before include "Type of onion" , "found in the allotment" , "Onion-like plant with clusters of bulbs" , "Rowing boat" , "Edible onion-like bulb" .)