Opening gin lethal to an extent (5)
I believe the answer is:
inlet
'opening' is the definition.
(inlet can mean a small opening)
'gin lethal to an extent' is the wordplay.
'to an extent' indicates the answer is hidden within the clue.
'INLET' is hidden within 'gin lethal'.
(Other definitions for inlet that I've seen before include "Place of entry" , "Small bay or creek" , "Narrow arm of water" , "Opening through which fluid can pass" , "Small area of sea" .)