One unclued light (three words) is a phrase referring to an item whose scientific name is formed by two unclued lights. The first word of this name can be divided into three words, each of which has two synonyms among the other unclued lights.
 

Across

1 Strip back stagecoach seat, not wide (5)

9 Check railway damage in station, causing weeping (10)

11 Artist reversed whole process (5)

14 Second-class designation sadly for red mineral (5)

15 Work in group with no time for sleep (5)

16 Jerks shout to encourage hounds (6)

21 Obtain picture showing slipper (8)

22 Support holding clamped valve (7)

24 Bore wanting new network...