Artist's photo to fool Oberleutnant at first (7)
I believe the answer is:
picasso
'artist's' is the definition.
(Picasso is an example)
'photo to fool oberleutnant at first' is the wordplay.
'photo' becomes 'pic' (I've seen this before).
'to fool' becomes 'ass' (ass is a kind of fool).
'at first' says to take the initial letters.
The initial letter of 'oberleutnant' is 'o'.
'pic'+'ass'+'o'='PICASSO'
(Other definitions for picasso that I've seen before include "Spanish artist who emigrated to France, d. 1973" , "See 1 Down" , "Spanish painter, d. 1973" , "Spanish painter and sculptor" , "Pablo . . . . . . ., extraordinary 20th century artist" .)