Turn on light to find explosive (9)
I believe the answer is:
gelignite
'explosive' is the definition.
(gelatin-based explosive substance)
'turn on light' is the wordplay.
'turn' says the letters should be written backwards.
'on' becomes 'leg' (cricket term, fielding positions).
'light' becomes 'ignite' (I've seen this in another clue).
'leg' in reverse letter order is 'gel'.
'gel'+'ignite'='GELIGNITE'
'to find' is the link.
(Other definitions for gelignite that I've seen before include "High explosive" , "(E.g. quarry) explosive" , "factor in altering mine?" , "source of flare-up" , "Tieing leg up in explosive" .)