Ancient working nurses led out (5)
I believe the answer is:
olden
'ancient' is the definition.
(as in the olden days)
'working nurses led out' is the wordplay.
'working' becomes 'on' (on can mean working or activated).
'nurses' is an insertion indicator (nurse can mean to hold onto or harbour).
'out' indicates an anagram (out can mean wrong or inaccurate).
'led' anagrammed gives 'lde'.
'on' placed around 'lde' is 'OLDEN'.
(Other definitions for olden that I've seen before include "of ancient history?" , "times gone by" , "Led on (anag)" , "Of a former age" , "Out of date" .)