Opening's curiously premature, losing millions (8)
I believe the answer is:
aperture
'opening's' is the definition.
(aperture is a kind of opening)
'curiously premature losing millions' is the wordplay.
'curiously' indicates an anagram.
'losing' suggests deleting specific letters.
'millions' becomes 'm' (abbreviation).
'premature' with 'm' taken away is 'preature'.
'preature' anagrammed gives 'APERTURE'.
(Other definitions for aperture that I've seen before include "Opening, to let light enter a camera perhaps" , "Crack" , "An opening that admits light" , "'Hole, opening (8)'" , "Eye" .)