Boil off that French skew (7)
I believe the answer is:
oblique
'skew' is the definition.
(synonyms)
'boil off that french' is the wordplay.
'off' indicates anagramming the letters.
'that french' becomes 'que' (French for 'that').
'boil' is an anagram of 'obli'.
'obli'+'que'='OBLIQUE'
(Other definitions for oblique that I've seen before include "Sloping; roundabout" , "Aslant" , "Indirect - devious" , "Slanted" , "This angle is obtuse or acute" .)