Appear to be a sound seam (4)
I believe the answer is:
seem
'appear to be' is the definition.
(I've seen this before)
'a sound seam' is the wordplay.
'a sound' shows a homophone (sound like).
'seam' is a homophone of 'SEEM'.
(Other definitions for seem that I've seen before include "Appear, look" , "Come across as" , "Appear, give impression of being" , "Appear to the eye" .)