Worker's outside clothes (6,4)
I believe the answer is:
boiler suit
'worker's' is the definition.
Both the definition and answer are singular nouns.
Maybe there's an association between them I don't understand?
'outside clothes' is the wordplay.
'outside' becomes 'boiler' (I can't justify this - if you can you should believe this answer much more).
'clothes' becomes 'suit' (set of clothes).
'boiler'+'suit'='BOILER SUIT'
Can you help me to learn more?
(Other definitions for boiler suit that I've seen before include "Protective clothing" , "Workwear" , "Overall" .)