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