Vigilant around leader of battalion, a Canadian province (7)
I believe the answer is:
alberta
'canadian province' is the definition.
(Alberta is an example)
'vigilant around leader of battalion a' is the wordplay.
'vigilant' becomes 'alert' ('alert' can be similar in meaning to 'vigilant').
'around' means one lot of letters goes inside another.
'leader of' says to take the initial letters.
The first letter of 'battalion' is 'b'.
'alert' enclosing 'b' is 'albert'.
'albert'+'a'='ALBERTA'
(Other definitions for alberta that I've seen before include "region of North America" , "Beat Lar up in Canada" , "Canadian province, capital Edmonton" , "Bale rat out of Canada" , "Einstein" .)