Small part of input for person given challenging task requiring password (6)
I believe the answer is:
sentry
'person given challenging task requiring password' is the definition.
(a sentry might ask for a password)
'small part of input' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'part of input' becomes 'entry' (part of what is entered).
's'+'entry'='SENTRY'
'for' is the link.
(Other definitions for sentry that I've seen before include "Watchman" , "Person standing watch" , "Picket" , "One who stands guard" , "Person posted to keep watch for some anticipated event" .)