Model mostly impressive, in my opinion (7)
I believe the answer is:
epitome
'model' is the definition.
(epitome is a kind of model)
'mostly impressive in my opinion' is the wordplay.
'mostly' means to remove the last letter.
'impressive' becomes 'epic' (I've seen this before).
'in my opinion' becomes 'tome' (to me).
'epic' with its last letter taken off is 'epi'.
'epi'+'tome'='EPITOME'
(Other definitions for epitome that I've seen before include "Paradigm" , "Something embodying perfectly a stated quality or idea" , "Perfect example of a quality" , "Exemplar - embodiment" , "Essence or embodiment" .)