Only half-like contrary jerk, which is perfectly legitimate (5)
I believe the answer is:
licit
'perfectly legitimate' is the definition.
'licit' can be an answer for 'legitimate' (synonyms). I am unsure of the 'perfectly' bit.
'only half-like contrary jerk' is the wordplay.
'only half' indicates taking half.
'contrary' is a reversal indicator.
'jerk' becomes 'tic' (jerking movement by the body).
'tic' written backwards gives 'cit'.
'like' cut in half is 'li'.
'li'+'cit'='LICIT'
'which is' acts as a link.
Can you help me to learn more?
(Other definitions for licit that I've seen before include "Not forbidden, lawful" , "Sanctioned" , "in order" , "Legitimate" , "Allowed as by law, not forbidden" .)