Best plump single mother (7)
I believe the answer is:
optimum
'best' is the definition.
('optimum' can be similar in meaning to 'best')
'plump single mother' is the wordplay.
'plump' becomes 'opt' (I've seen this before).
'single' becomes 'i' (one).
'mother' becomes 'mum' (mum is a kind of mother).
'opt'+'i'+'mum'='OPTIMUM'
(Other definitions for optimum that I've seen before include "model" , "Best, most favourable" , "Most favourable point" , "Most favourable amount" , "The most favourable condition" .)