Spike welcomes surprisingly nice pacifist (8)
I believe the answer is:
peacenik
'pacifist' is the definition.
(peacenik is a kind of pacifist)
'spike welcomes surprisingly nice' is the wordplay.
'spike' becomes 'peak' (I can't explain this - if you can you should believe this answer much more).
'welcomes' indicates putting letters inside (in sense of 'invites in').
'surprisingly' is an anagram indicator (a surprising ordering of the letters).
'nice' with letters rearranged gives 'ceni'.
'peak' going around 'ceni' is 'PEACENIK'.
Can you help me to learn more?
(Other definitions for peacenik that I've seen before include "Aldermaston marcher?" , "Pacifist" .)