Traffic merging, say - it requires a lot of workers (5,5)
I believe the answer is:
trade union
'it requires a lot of workers' is the definition.
(trade unions are made up of workers)
'traffic merging say' is the wordplay.
'traffic' becomes 'trade' (I've seen this before).
'merging say' becomes 'union' (a union is a merging of people or things).
'trade'+'union'='TRADE-UNION'
(Other definitions for trade union that I've seen before include "Organised Labour" , "Equity is one such" , "Equity perhaps" .)