Allen screws holding optical glass (4)
I believe the answer is:
lens
'optical glass' is the definition.
(I've seen this in another clue)
'allen screws holding' is the wordplay.
'holding' indicates a hidden word.
'LENS' is hidden in the letters of 'allen screws'.
(Other definitions for lens that I've seen before include "Piece of glass, usually curved" , "Part of photographer's equipment" , "Dark cloud; shining light" , "French town" , "Device to converge or diverge light" .)