Understand you must quote the letter (3)
I believe the answer is:
'understand' is the definition.
(I know that understand can be written as see)
'quote the letter' is the wordplay.
'quote' shows a homophone (sound like) (I've seen 'quoted' mean this).
'the letter' becomes 'c'.
'c' is a homophone of 'SEE'.
'you must' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for see that I've seen before include "Jurisdiction of a bishop" , "Observe the bishop's domain" , "Watch, equal (a bet)" , "Salisbury, for one" , "Get < court < witness" .)