Witness reads letter aloud (3)
I believe the answer is:
see
'witness reads' is the definition.
'see' can be an answer for 'witness' (synonyms). I'm not certain of the 'reads' bit.
'letter aloud' is the wordplay.
'letter' becomes 'c'.
'aloud' indicates a 'sounds like' (homophone) clue.
'c' is a homophone of 'SEE'.
(Other definitions for see that I've seen before include "Watch, equal (a bet)" , "View; prelacy" , "Discern visually" , "Consider; diocese" , "View; realise" .)