Millions invested in bags for fishing vessels (6)
I believe the answer is:
smacks
'fishing vessels' is the definition.
(I've seen this before)
'millions invested in bags' is the wordplay.
'millions' becomes 'm' (abbreviation).
'invested in' indicates putting letters inside.
'bags' becomes 'sacks' (sack is a kind of bag).
'm' inserted inside 'sacks' is 'SMACKS'.
'for' is the link.
(Other definitions for smacks that I've seen before include "Small fishing vessels" , "Strikes with the hand" , "Fishing ships with sails" , "Smells; fishing boats" , "Horse's" .)