Appear to be a sound seam (4)
I believe the answer is:
seem
'appear to be' is the definition.
(I've seen this in another clue)
'a sound seam' is the wordplay.
'a sound' indicates a 'sounds like' (homophone) clue.
'seam' sounds like '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" .)