Plump single mother, ideal (7)
I believe the answer is:
optimum
'ideal' is the definition.
(thesaurus)
'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 "Best (result)" , "Most appropriate" , "top condition" , "model" , "Most favourable amount" .)