In skill the French sentry should be (5)
I believe the answer is:
alert
'sentry should be' is the definition.
I can't tell whether this defines the answer.
'in skill the french' is the wordplay.
'in' means one lot of letters goes inside another.
'skill' becomes 'art' (both can mean a talent).
'the french' becomes 'le' ('the' in French).
'art' placed around 'le' is 'ALERT'.
Can you help me to learn more?
(Other definitions for alert that I've seen before include "Prompt" , "sharp-witted" , "On one's toes" , "In state of watchfulness" , "Watchfully awake" .)