Join Frenchman following the waves (4)
I believe the answer is:
seam
'join' is the definition.
(I know that seam is a more specific form of join)
'frenchman following the waves' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur).
'following' means one lot of letters go next to another.
'the waves' becomes 'sea' (the sea is full of waves).
'm' after 'sea' is 'SEAM'.
(Other definitions for seam that I've seen before include "Join in knitted garment" , "Underground layer of mineral" , "The line where two edges are stitched together" , "Line of join" , "Join in material - layer of coal" .)