Magnates, oddly, dismissed workers (4)
I believe the answer is:
ants
'workers' is the definition.
(I've seen this before)
'magnates oddly dismissed' is the wordplay.
'oddly dismissed' indicates alternate letters (odd-numbered letters get removed).
The alternate letters of 'magnates' are 'ANTS'.
(Other definitions for ants that I've seen before include "Industrious insects" , "Tiny insects that live in colonies" , "workers found returning" , "Emmets" , "Busy little insects" .)