Checked up to find it’s a unit of weight (6)
I believe the answer is:
denier
'find it's a unit of weight' is the definition.
I can't tell whether this defines the answer.
'checked up' is the wordplay.
'checked' becomes 'reined' (to rein something in is to control or check it).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'reined' backwards is 'DENIER'.
'to' is the link.
Can you help me to learn more?
(Other definitions for denier that I've seen before include "Person refusing to admit (something)" , "how fine stockings are?" , "One refusing to accept" , "One who doesn't believe" , "One refusing to face facts" .)