Signal from visitor dropping union leader on river (7)
I believe the answer is:
gesture
'signal' is the definition.
(gesture is a kind of signal)
'visitor dropping union leader on river' is the wordplay.
'visitor' becomes 'guest' (guest is a kind of visitor).
'dropping' is a deletion indicator.
'union' becomes 'u' (common abbreviation as in 'Trades Union Congress').
'leader on river' becomes 'ure' (I can't explain this - if you can you should give a lot more credence to this answer).
'guest' with 'u' removed is 'gest'.
'gest'+'ure'='GESTURE'
'from' acts as a link.
(Other definitions for gesture that I've seen before include "Hand movement to communicate" , "Message through movement" , "wave, perhaps?" , "Eg, shrug or wave" , "Movement to express meaning" .)