Surprised expression when business comes to nothing (3)
I believe the answer is:
coo
'surprised expression' is the definition.
(I've seen this before)
'business comes to nothing' is the wordplay.
'business' becomes 'co'.
'comes to' is a charade indicator (letters next to each other).
'nothing' becomes 'o' (looks like zero - 0**).
'co'+'o'='COO'
'when' is the link.
(Other definitions for coo that I've seen before include "(Of pigeon) murmur" , "Pigeon call" , "crumbs" , "Speak fondly" , "Murmur amorously" .)