Crossword Genius

It's said to give a degree of restraint (4)

Ross

I believe the answer is:

rein

I'm a little stuck... Click here to teach me more about this clue!

'restraint' is the definition.
(I've seen this before)

'it's said to give a degree' is the wordplay.
I cannot really understand how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is present in the answer.
The remaining letters 'rin' 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).

'of' is the link.

Can you help me to learn more?

(Other definitions for rein that I've seen before include "leading article?" , "given free to experienced riders?" , "Leather used for control in horse-riding" , "Restraint; strap attached to horse's bit" , "A means of control" .)

I've seen this clue in the Evening Standard.
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