Millions can be seen on main vein (4)
I believe the answer is:
seam
'vein' is the definition.
(I've seen this in another clue)
'millions can be seen on main' is the wordplay.
'millions' becomes 'm' (abbreviation).
'can be seen on' means one lot of letters go next to another.
'main' becomes 'sea' (the main is an old name for the sea).
'm' after 'sea' is 'SEAM'.
(Other definitions for seam that I've seen before include "The line where two edges are stitched together" , "Joint between two edges" , "Join round cricket ball" , "Stratum (of a mineral)" , "Sewn join" .)