French girl left Channel Islands, heading north (6)
I believe the answer is:
gallic
'french' is the definition.
(I've seen this before)
'girl left channel islands heading north' is the wordplay.
'girl' becomes 'gal' ('gal' can be a synonym of 'girl'**).
'left' becomes 'l' (common abbreviation).
'channel islands' becomes 'ci'.
'heading north' shows that the letters should be reversed in order.
'ci' backwards is 'ic'.
'gal'+'l'+'ic'='GALLIC'
(Other definitions for gallic that I've seen before include "Of the French/France" , "Typically French or of France" , "Like certain wars" , "Pertaining to France" , "Characteristic of the French" .)