Violet turns up unknown photo (5)
I believe the answer is:
pansy
'violet' is the definition.
(pansy violet is a kind of violet)
'turns up unknown photo' is the wordplay.
'turns up' shows that the letters should be reversed in order.
'unknown' becomes 'y' (algebra).
'photo' becomes 'snap' (I've seen this before).
'y'+'snap'='ysnap'
'ysnap' backwards is 'PANSY'.
(Other definitions for pansy that I've seen before include "Cultivated viola" , "Garden plant with large flowers of various colours" , "Species of violet" , "Rich-coloured viola" , "Type of viola" .)