Terribly tired without right food regime (4)
I believe the answer is:
diet
'food regime' is the definition.
(I've seen this in another clue)
'terribly tired without right' is the wordplay.
'terribly' indicates an anagram.
'without' suggests deleting specific letters.
'right' becomes 'r' (common abbreviation).
'tired' with 'r' taken away is 'tied'.
'tied' with letters rearranged gives 'DIET'.
(Other definitions for diet that I've seen before include "Supposed slimming plan" , "regular intake" , "Restrict food-intake" , "A restricted eating regime" , "control weight" .)