What could lead whistleblower to share photos? (6)
I believe the answer is:
prefix
'what could lead' is the definition.
(prefixes appear at the beginning of words)
'whistleblower to share photos?' is the wordplay.
'whistleblower' becomes 'ref' (referee).
'to share' indicates putting letters inside (inserted letters share some space within others).
'photos?' becomes 'pix' (short for pictures).
'ref' put into 'pix' is 'PREFIX'.
(Other definitions for prefix that I've seen before include "Part added to front of a word" , "Beginning part of a word" , "add a bit on" , "Word added in front of another" , "initial qualifier" .)