It has a beastly restricting effect (6)
I believe the answer is:
tether
'a beastly restricting effect' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'it' is the wordplay.
I cannot really understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
The remaining letters 'ether' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'has' acts as a link.
Can you help me to learn more?
(Other definitions for tether that I've seen before include "Rope holding an animal" , "Rope; the exasperated at the end of it" , "restricts movement" , "Leash" , "Hold by a rope" .)