Key picture is very impressive (4)
I believe the answer is:
epic
'very impressive' is the definition.
(I've seen this before)
'key picture' is the wordplay.
'key' becomes 'e' (I've seen this in other clues).
'picture' becomes 'pic'.
'e'+'pic'='EPIC'
'is' acts as a link.
(Other definitions for epic that I've seen before include "Larger than life" , "Large-scale" , "great film" , "imposing" , "Large in scale" .)