Protest involving one boarding small vessel (3,2)
I believe the answer is:
sit in
'protest' is the definition.
(sit-in is a type of non-violent protest)
'one boarding small vessel' is the wordplay.
'one' becomes 'i' (Roman numeral).
'boarding' is an insertion indicator.
'small' becomes 's' (abbreviation - e.g. clothes size).
'vessel' becomes 'tin' (I've seen this in other clues).
's'+'tin'='stin'
'i' going inside 'stin' is 'SIT-IN'.
'involving' is the link.
(Other definitions for sit in that I've seen before include "form of protest" , "act as someone's deputy" , "Act as a substitute for" , "demonstration" .)