Like rosin, a treatment for reins (5)
I believe the answer is:
resin
'like rosin' is the definition.
'resin' can be an answer for 'rosin' (I have seen 'eg Rosin ' mean 'resin' so perhaps 'rosin' could also mean 'resin'). I am not certain of the 'like' bit.
'a treatment for reins' is the wordplay.
'a treatment for' indicates anagramming the letters (I've seen this in other clues).
'reins' is an anagram of 'RESIN'.
Can you help me to learn more?
(Other definitions for resin that I've seen before include "Sticky material" , "viscous material" , "Glue" , "varnish, perhaps" , "Sticky exudation from trees" .)