A hundred are on the vessel to stroke (6)
I believe the answer is:
caress
'to stroke' is the definition.
(I know that caress is a more specific form of the action stroke)
'a hundred are on the vessel' is the wordplay.
'a hundred' becomes 'C' (Roman numeral).
'on' means one lot of letters go next to another.
'the vessel' becomes 'SS' (prefix in ship names eg SS Great Britain).
'c'+'are'+'ss'='CARESS'
(Other definitions for caress that I've seen before include "show of affection" , "Scares with loving gesture" , "Stroke, fondle" , "Stroke gently" , "Pat" .)