Nothing to be gained by mixing untrained workers together (5,5)
I believe the answer is:
trade union
'workers together' is the definition.
(union of workers)
'nothing to be gained by mixing untrained' is the wordplay.
'nothing' becomes 'O' (O resembles 0).
'to be gained by' is an insertion indicator (inserted letters are acquired or gained by others).
'mixing' is an anagram indicator.
'untrained' with letters rearranged gives 'tradeunin'.
'o' inserted within 'tradeunin' is 'TRADE-UNION'.
(Other definitions for trade union that I've seen before include "Equity perhaps" , "Organised Labour" , "Equity is one such" .)