Very old worker consumed nice crackers (7)
I believe the answer is:
ancient
'very old' is the definition.
(I've seen this before)
'worker consumed nice crackers' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'consumed' means one lot of letters goes inside another.
'crackers' indicates an anagram (informal term for mad).
'nice' with letters rearranged gives 'cien'.
'ant' enclosing 'cien' is 'ANCIENT'.
(Other definitions for ancient that I've seen before include "From the distant past" , "Archaic" , "from Aristophanes' time" , "Veteran" , "Very old or belonging to times past" .)