Old worker hides nice arrangement (7)
I believe the answer is:
ancient
'old' is the definition.
(similar in meaning)
'worker hides nice arrangement' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'hides' means one lot of letters goes inside another.
'arrangement' indicates anagramming the letters.
'nice' is an anagram of 'cien'.
'ant' enclosing 'cien' is 'ANCIENT'.
(Other definitions for ancient that I've seen before include "Dating from long ago" , "Antiquated" , "so old-fashioned" , "From the distant past" , "very old boy?" .)