What is that word that is used to say someone "takes advantage of the situation". This word usually has a mildly negative connotation. It is related to being clever.
The word closest to that definition is probably opportunist, although historically it has been used in a rather specialised sense.
I feel that the word most closely associated with taking advantage of a situation is exploit which also carries a slightly negative connotation.
Have you considered Exploitative?
exploiting or tending to exploit; especially : unfairly or cynically using another person or group for profit or advantage
"exploitative terms of employment"
"an exploitative film"
Merriam-Webster has its first-known use as 1885.
Cunning, which according to OED means:
1 having or showing skill in achieving one's ends by deceit or evasion: a cunning look came into his eyes.
Capitalise - draw advantages from; "he is capitalizing on her mistake"; "she took advantage of his absence to meet her lover"
For a word with less negative connotations, I would suggest the word astute.
Here's the definition from oxforddictionaries.com:
having or showing an ability to accurately assess situations or people and turn this to one’s advantage: an astute businessman