One unclued light is a three-word phrase indicating the way in which each of the answers to clues in italics must be entered in the grid. Definitions of resulting entries are supplied by unclued lights.
 

Across

1    Fissure filled with new toxic substance
12    See a signal, admitting hesitation, only returning for electronic device (10, two words)
13    Nymph’s love declared (5)
14    Habitual response in part I concealed
15    Correct about actress Welch producing crazed effect (10)
16    Fatigue involved in feverish attack (7, hyphened)
22    Come into trendy hotel with revolutionary flag (7)
23    Artist, minor, lacking power (4)
24   ...