Reckon to eat poorly at various times (8)
I believe the answer is:
estimate
'reckon' is the definition.
(estimating is a kind of reckoning)
'eat poorly at various times' is the wordplay.
'poorly' indicates anagramming the letters.
'at' says to put letters next to each other (I've seen this in other clues).
'various' is an anagram indicator.
'eat' with letters rearranged gives 'ate'.
'times' anagrammed gives 'estim'.
'ate' after 'estim' is 'ESTIMATE'.
'to' acts as a link.
(Other definitions for estimate that I've seen before include "Guess the cost" , "Calculate approximately" , "Tease Tim (anag.)" , "Reckon value" , "Meatiest (anag) - figure" .)