Drunk embracing heartless lass rocks biannual event (8)
I believe the answer is:
solstice
'biannual event' is the definition.
(the winter and summer solstices)
'drunk embracing heartless lass rocks' is the wordplay.
'drunk' becomes 'sot' (both can mean someone who drinks too much).
'embracing' means one lot of letters goes inside another.
'heartless' suggests removing the centre.
'rocks' becomes 'ice' (both can informally mean diamonds).
'lass' with its middle removed is 'ls'.
'sot' placed around 'ls' is 'solst'.
'solst'+'ice'='SOLSTICE'
(Other definitions for solstice that I've seen before include "Closest I come to time when sun is farthest from equator" , "High or low point for the sun" , "Shortest or longest day of the year" , "When the sun is farthest from the celestial equator" , "Time of year when sun is furthest from equator" .)