Peruse a letter in red (4)
I believe the answer is:
read
'peruse' is the definition.
(I've seen this before)
'a letter in red' is the wordplay.
'a letter' becomes 'a'.
'in' indicates putting letters inside.
'a' placed into 'red' is 'READ'.
(Other definitions for read that I've seen before include "Studied" , "Comprehend written words" , "(Of passage) have a certain wording" , "Examine" , "Undertake a course of study" .)