Old worker eating nice bananas (7)
I believe the answer is:
ancient
'old' is the definition.
(similar in meaning)
'worker eating nice bananas' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'eating' indicates putting letters inside.
'bananas' is an anagram indicator (informal term for mad).
'nice' is an anagram of 'cien'.
'ant' enclosing 'cien' is 'ANCIENT'.
(Other definitions for ancient that I've seen before include "Aged and timeworn" , "so old-fashioned" , "Early" , "Belonging to a former age" , "Of times long past" .)