Ones who do sums to slice when wastage occurs, finding turning point (6,8)
I believe the answer is:
summer solstice
'turning point' is the definition.
'ones who do sums to slice when wastage occurs' is the wordplay.
'ones who do sums' becomes 'summers'.
'when wastage occurs' indicates an anagram.
'to'+'slice'='toslice'
'toslice' with letters rearranged gives 'olstice'.
'summers'+'olstice'='SUMMER SOLSTICE'
'finding' is the link.
(Other definitions for summer solstice that I've seen before include "one day in June" , "Time of longest day" .)