Lacking politeness, if innocuous — merely unsettling (15)
I believe the answer is:
unceremoniously
'lacking politeness' is the definition.
(in a way which is impolite or informal)
'innocuous merely unsettling' is the wordplay.
'unsettling' indicates anagramming the letters.
'innocuous'+'merely'='innocuousmerely'
'innocuousmerely' with letters rearranged gives 'UNCEREMONIOUSLY'.
'if' acts as a link.
(Other definitions for unceremoniously that I've seen before include "on the spot" , "Without further ado" , "Abruptly" .)