Head of snake, more sinuous snake (7)
I believe the answer is:
slither
'snake' is the definition.
(I've seen this before)
'head of snake more sinuous' is the wordplay.
'head of' says to take the initial letters.
'more sinuous' becomes 'lither'.
The initial letter of 'snake' is 's'.
's'+'lither'='SLITHER'
(Other definitions for slither that I've seen before include "Slide, slip" , "Undulate" , "Slide (like a snake)" , "Slide on slippery surface" , "Move smoothly over a surface with a twisting motion" .)