vigilant
attentive, cautious, careful, wakeful, keen, watchful, alert, guarded, acute
chocolate
Dull.
No. The suffix -ful means "full" but is only a compound noun in such words as cupful or teaspoonful. In watchful, peaceful, and beautiful, it just forms an adjective.
Synonyms for cautious: watchful alert safe careful
There is no anagram for the word watchful. The next longest possible word from those letters is hatful.
The word "lean" fits your criteria as it means cool and rhymes with green.
The word "keen" rhymes with mean and can also mean to scream loudly or shout enthusiastically.
The security guard remained watchful throughout the night, ensuring the safety of the premises.
alert
The word watch can be a noun and a verb. The noun is a device that tells the time worn on the wrist. The verb form means to observe.
Sharp ANSWER 2 Keen means intelligent. It can also be a verb and mean to wail or sob loudly. For example: "The woman saw that her child was dead and began to keen." Keen is also a slang word from the 50's and 60's and it means great, wonderful, good, cool. A teenager might remark, " That is a keen guitar you have."