Ascertain Dicky is a member of narrow group (9)
I believe the answer is:
sectarian
'a member of narrow group' is the definition.
(member of a sect)
'ascertain dicky' is the wordplay.
'dicky' is an anagram indicator (dicky can mean ill or non-functioning).
'ascertain' is an anagram of 'SECTARIAN'.
'is' acts as a link.
(Other definitions for sectarian that I've seen before include "Prejudiced against other religions? So ascertain" , "Concerning one group" , "Ascertain to be with religious prejudice" , "Adhering to a faction or doctrine" , "from an unorthodox group" .)