Look at a letter, by the sound of it (3)
I believe the answer is:
see
'look at' is the definition.
(I've seen this in another clue)
'a letter by the sound of it' is the wordplay.
'a letter' becomes 'c'.
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'c' sounds like 'SEE'.
(Other definitions for see that I've seen before include "Perceive a bishopric" , "Envisage; ensure" , "Understand; view" , "Observe a bishopric" , "Bishop's diocese" .)