Appear to join, say (4)
I believe the answer is:
seem
'appear' is the definition.
(I know that seem is a more specific form of appear)
'join say' is the wordplay.
'join' becomes 'seam' (seaming is a kind of joining).
'say' indicates a 'sounds like' (homophone) clue.
'seam' sounds like 'SEEM'.
'to' acts as a link.
(Other definitions for seem that I've seen before include "Come across as" , "Appear to the eye" , "Appear, give impression of being" , "Appear, look" .)