Fancy large model (5)
I believe the answer is:
ideal
'model' is the definition.
(ideal can mean model or perfect)
'fancy large' is the wordplay.
'fancy' becomes 'idea' (I've seen this before).
'large' becomes 'L' (eg in clothes sizes).
'idea'+'l'='IDEAL'
(Other definitions for ideal that I've seen before include "unlikely to be bettered?" , "Just perfect, couldn't be better" , "Model of ethical behaviour" , "without flaws" , "Optimal" .)