Guard has to be careful (7)
I believe the answer is:
lookout
'guard' is the definition.
(I know that lookout is a type of security guard)
'to be careful' is the wordplay.
'to be' becomes 'look' (looking is a kind of being).
'careful' becomes 'out' (I can't explain this - if you can you should give a lot more credence to this answer).
'look'+'out'='LOOKOUT'
'has' acts as a link.
Can you help me to learn more?
(Other definitions for lookout that I've seen before include "Person keeping watch" , "Guard on duty" , "One keeping watch" , "Observatory" , "Outlook for sentry" .)