Stern and bow of ship going in one sudden movement (7)
I believe the answer is:
ascetic
'stern' is the definition.
(thesaurus)
'bow of ship going in one sudden movement' is the wordplay.
'bow of' says to take the initial letters (front of).
'going in' means one lot of letters goes inside another.
'one' becomes 'ace' (ace has a value of one in some card games).
'sudden movement' becomes 'tic' (an involuntary physical movement).
The first letter of 'ship' is 's'.
's' placed within 'ace' is 'asce'.
'asce'+'tic'='ASCETIC'
'and' is the link.
(Other definitions for ascetic that I've seen before include "Self-denying, not allowing oneself luxuries" , "Abstemious, self-denying" , "Self-disciplined person" , "Austere, abstinent" , "Eremite" .)