Model one found in plump old lady (7)
I believe the answer is:
optimum
'model' is the definition.
(I've seen this before)
'one found in plump old lady' is the wordplay.
'one' becomes 'i' (Roman numeral).
'found in' indicates putting letters inside.
'plump' becomes 'opt' (I've seen this before).
'old lady' becomes 'mum' (I've seen this before).
'opt'+'mum'='optmum'
'i' inserted within 'optmum' is 'OPTIMUM'.
(Other definitions for optimum that I've seen before include "Most appropriate" , "The best possible" , "Most favourable (conditions)" , "Most favourable point" , "Best, most favourable" .)