Start part of play that's about large vessel (8)
I believe the answer is:
activate
'start' is the definition.
(I've seen this before)
'part of play that's about large vessel' is the wordplay.
'part of play' becomes 'act' ('act' is part of 'play').
'that's' becomes 'ie'.
'about' indicates putting letters inside.
'large vessel' becomes 'vat' (I've seen this before).
'ie' placed around 'vat' is 'ivate'.
'act'+'ivate'='ACTIVATE'
(Other definitions for activate that I've seen before include "Put into operation" , "Launch" , "Start working" , "Switch on" , "Cause to move or start" .)