Best time with one mum after work (7)
I believe the answer is:
optimum
'best' is the definition.
(optimum means the best possible)
'time with one mum after work' is the wordplay.
'time' becomes 't' (abbreviation).
'with' is a charade indicator (letters next to each other).
'one' becomes 'I' (Roman numeral).
'after' is a charade indicator (letters next to each other).
'work' becomes 'op' (abbreviation for opus).
'i'+'mum'='imum'
't' put next to 'imum' is 'timum'.
'timum' after 'op' is 'OPTIMUM'.
(Other definitions for optimum that I've seen before include "top condition" , "Best, most favourable" , "Most favourable amount" , "The best possible" , "The most favourable condition" .)