One contract for model (5)
I believe the answer is:
ideal
'model' is the definition.
(ideal is a kind of model)
'one contract' is the wordplay.
'one' becomes 'I' (Roman numeral).
'contract' becomes 'deal' (I've seen this in another clue).
'i'+'deal'='IDEAL'
'for' is the link.
(Other definitions for ideal that I've seen before include "The best conceivable" , "Just perfect, couldn't be better" , "Standard of perfection or excellence" , "Model of ethical behaviour" , "immaculate" .)