Sentry wrong to support King John at the front (7)
I believe the answer is:
lookout
'sentry' is the definition.
(I know that sentry can be written as lookout)
'wrong to support king john at the front' is the wordplay.
I cannot quite see how this works, but
'wrong' could be 'out' (eg a calculation might be 'out') and 'out' is found within the answer.
'king' could be 'k' (abbreviation for king in chess) and 'k' is found within the answer.
'john' could be 'loo' (loo is a kind of john) and 'loo' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for lookout that I've seen before include "Observer; concern" , "One keeping watch" , "Person keeping watch" , "Watchman" , "Watchtower" .)