Plump mother protecting one best (7)
I believe the answer is:
optimum
'best' is the definition.
('optimum' can be similar in meaning to 'best')
'plump mother protecting one' is the wordplay.
'plump' becomes 'opt' (I've seen this before).
'mother' becomes 'mum' (mum is a kind of mother).
'protecting' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'opt'+'mum'='optmum'
'optmum' placed around 'i' is 'OPTIMUM'.
(Other definitions for optimum that I've seen before include "Most favourable point" , "Best, most favourable" , "Most appropriate" , "Most favourable (conditions)" , "Best (result)" .)