Best plump for single parent (7)
I believe the answer is:
optimum
'best' is the definition.
(optimum means the best possible)
'plump for single parent' is the wordplay.
'plump for' becomes 'opt' (I've seen this before).
'single' becomes 'I' (one).
'parent' becomes 'mum' (mum means a mother).
'opt'+'i'+'mum'='OPTIMUM'
(Other definitions for optimum that I've seen before include "The most favourable condition" , "Best (result)" , "Best, most favourable" , "The best possible" , "model" .)