Arouse old jerk returning by Eurostar at first (6)
I believe the answer is:
excite
'arouse' is the definition.
(I know that arouse is a more specific form of excite)
'old jerk returning by eurostar at first' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'jerk' becomes 'tic' (jerking movement by the body).
'returning' shows that the letters should be reversed in order.
'by' means one lot of letters go next to another.
'at first' says to take the initial letters.
The first letter of 'eurostar' is 'e'.
'tic' back-to-front is 'cit'.
'ex'+'cit'+'e'='EXCITE'
(Other definitions for excite that I've seen before include "Turn on" , "Stir emotionally" , "Cause (someone) to feel very enthusiastic" , "Thrill" , "Give rise to (a feeling)" .)