Order to have noticeable effect (4)
I believe the answer is:
tell
This clue could be a double definition.
'order' is the first definition.
(I know that order can be written as tell)
'have noticeable effect' is the second definition.
'tell' can be an answer for 'have' (thesaurus). I'm not sure about the 'noticeable effect' bit.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for tell that I've seen before include "Rossini composed opera 'William ....'" , "Instruct" , "Order" , "Grass - Swiss hero" , "... national hero" .)