Pitcher starts to tip, hampering one moving vessel (7)
I believe the answer is:
thrower
'pitcher' is the definition.
'starts to tip hampering one moving vessel' is the wordplay.
'starts to' indicates taking the first letters.
'one moving vessel' becomes 'rower' (person who moves a vessel).
The initial letters of 'tip hampering' is 'th'.
'th'+'rower'='THROWER'
(Other definitions for thrower that I've seen before include "One who intentionally loses" , "tosser" , "who offends the umpire?" , "shyer" , "participant in discus event?" .)