To restrain is far different from 'refrain' (4)
I believe the answer is:
rein
'to restrain' is the definition.
(reining is a kind of restraining)
'far different from refrain' is the wordplay.
I cannot quite understand how this works, but
'from' could be 'in' (I've seen this in other clues) and 'in' is found in the answer.
The remaining letters 're' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for rein that I've seen before include "(Horse's) guiding strap" , "Strap on a bridle" , "Director's power" , "Restraining lead" , "Curb" .)