He's on the watch for the Second Coming (6)
I believe the answer is:
sentry
'he's on the watch' is the definition.
'sentry' can be an answer for 'watch' (synonyms). I am unsure of the 'he's on the' bit.
'second coming' 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 see.
This explanation may well be incorrect...
'for the' is the link.
Can you help me to learn more?
(Other definitions for sentry that I've seen before include "One who stands guard" , "Lookout person" , "Mil. guard" , "Watchman" , "One keeps watch" .)