Might associated with underworld boss causing alarm (6)
I believe the answer is:
dismay
'alarm' is the definition.
(synonyms)
'might associated with underworld boss' is the wordplay.
'might' becomes 'may' (I've seen this before).
'associated with' is a charade indicator (letters next to each other).
'underworld boss' becomes 'dis' (Roman god of the underworld).
'may' put after 'dis' is 'DISMAY'.
'causing' acts as a link.
(Other definitions for dismay that I've seen before include "Feeling of surprise and discouragement" , "Consternation, distress" , "Alarm, apprehension" , "'Alarm, consternation (6)'" , "Consternation, discouragement" .)