Photograph of crack marksman
I believe the answer is:
snapshot
'photograph' is the definition.
(snapshot is a kind of photograph)
'crack marksman' is the wordplay.
'crack' becomes 'snap' ('snap' can be a synonym of 'crack'**).
'marksman' becomes 'shot' (I've seen this in other clues).
'snap'+'shot'='SNAPSHOT'
'of' acts as a link.
(Other definitions for snapshot that I've seen before include "Instant record of an event" , "Quick photograph" , "Pan hosts over casual photo" , "Informal photo" , "Picture" .)