Second appearance for guard (6)
I believe the answer is:
sentry
'for guard' is the definition.
(sentry is a kind of guard)
'second appearance' is the wordplay.
I cannot quite see how this works, but
'second' could be 's' (common abbreviation) and 's' is found within the answer.
The remaining letters 'entry' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sentry that I've seen before include "Military guard" , "Mil. guard" , "Watchperson" , "Watchman" , "Sentinel, look-out" .)