Best work only half the time, mother (7)
I believe the answer is:
optimum
'best' is the definition.
(optimum means the best possible)
'work only half the time mother' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'only half' suggests halving the letters.
'mother' becomes 'mum' (informal term for mother).
'time' cut in half is 'ti'.
'op'+'ti'+'mum'='OPTIMUM'
(Other definitions for optimum that I've seen before include "The best possible" , "Most favourable point" , "Best, most favourable" , "Most favourable amount" , "The most favourable condition" .)