The first one who practises magic has a nervous jerk (6)
I believe the answer is:
twitch
'a nervous jerk' is the definition.
'twitch' can be an answer for 'jerk' (I've seen this before). I'm not sure about the 'a nervous' bit.
'the first one who practises magic' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'first' says to take the initial letters.
'practises magic' becomes 'itch' (I am not sure about this - if you are sure you should believe this answer much more).
The first letter of 'who' is 'w'.
't'+'w'+'itch'='TWITCH'
'has' is the link.
Can you help me to learn more?
(Other definitions for twitch that I've seen before include "One may be nervous" , "Jerk suddenly" , "Short sudden jerking movement" , "Quiver or contract spasmodically, as eye may" , "jump" .)