Discover by chance (3,2)
I believe the answer is:
hit on
'discover by chance' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hit on that I've seen before include "Discover; single out" , "try a pass?" , "Discover, think of" , "Discovered suddenly or, in modern slang, made advances towards" , "Try to get off with" .)