A few supposed greeting to be for an unknown reason (7)
I believe the answer is:
somehow
'be for an unknown reason' is the definition.
(for some reason)
'a few supposed greeting' is the wordplay.
'a few' becomes 'some' (both can mean a small amount of something).
'supposed greeting' becomes 'how' (greeting used by some Native Americans).
'some'+'how'='SOMEHOW'
'to' acts as a link.
(Other definitions for somehow that I've seen before include "For an unknown reason; so who, me? (anag.)" , "By one means or another" , "By unspecified means" , "For some unspecified reason" , "One way or another" .)