Sailor with external support gets new beginning (7)
I believe the answer is:
restart
'gets new beginning' is the definition.
(to restart is to begin again)
'sailor with external support' is the wordplay.
'sailor' becomes 'tar' (slang term for a sailor).
'with external' indicates putting letters inside.
'support' becomes 'rest' (e.g. a shoulder rest is a support for shoulder).
'tar' put into 'rest' is 'RESTART'.
(Other definitions for restart that I've seen before include "launch again" , "Resume" , "begin afresh" , "Commence again" , "Jumping the gun will mean" .)