Underworld boss's place you went to finally (5)
I believe the answer is:
pluto
'underworld boss's' is the definition.
(Roman god of the underworld)
'place you went to finally' is the wordplay.
'place' becomes 'pl' (abbreviation used in postal addresses).
'finally' suggests the final letters.
The final letters of 'you went to' are 'uto'.
'pl'+'uto'='PLUTO'
(Other definitions for pluto that I've seen before include "it's another world there" , "Ninth planet of solar system" , "Planet beyond Neptune" , "underworld boss" , "Outermost of the main planets" .)