The most common and most likely answer to this crossword clue is the 7 letter word OPERANT
O
P
E
R
A
N
T
More clues leading to the result OPERANT
Buddy explains!

The straight part
We know that "effective" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case effective = operant.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does OPERANT mean?
OPERANT noun- An operative person or thing.
- Any of a class of behaviors that produce consequences by operating (i.e., acting) upon the environment.
- That operates to produce an effect.