Old worker has nice arrangement (7)
I believe the answer is:
ancient
'old' is the definition.
('ancient' can be similar in meaning to 'old')
'worker has nice arrangement' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'has' indicates putting letters inside.
'arrangement' indicates anagramming the letters.
'nice' anagrammed gives 'cien'.
'ant' going around 'cien' is 'ANCIENT'.
(Other definitions for ancient that I've seen before include "Historic standard" , "Pistol" , "no longer current" , "Very old, like some civilizations" , "Antediluvian" .)