Craft starts to speed leaving unknown headland (7)
I believe the answer is:
slyness
'craft' is the definition.
('slyness' can be a synonym of 'craft')
'starts to speed leaving unknown headland' is the wordplay.
'starts to' indicates taking the first letters.
'unknown' becomes 'y' (algebra).
'headland' becomes 'ness' (I've seen this before).
The first letters of 'speed leaving' is 'sl'.
'sl'+'y'+'ness'='SLYNESS'
(Other definitions for slyness that I've seen before include "Cunning behaviour" , "Cunning; guile" .)