Effective (7)
'effective' is the definition.
(synonyms)
This is all the clue.
(Other definitions for telling that I've seen before include "Striking, having noticeable effect" , "Powerful - effective" , "Giving an order" , "briefing" , "relation" .)
'effective' is the definition.
(I know that effective can be written as efficacious)
This is all the clue.
(Other definitions for efficacious that I've seen before include "working" , "able to produce the right result" , "producing the desired result" , "Effective" .)