Template for drawing nicest wavy line (7)
I believe the answer is:
stencil
'template for drawing' is the definition.
'nicest wavy line' is the wordplay.
'wavy' indicates anagramming the letters (letters wave around).
'line' becomes 'l' (used when specifying particular lines from a poem).
'nicest' anagrammed gives 'stenci'.
'stenci'+'l'='STENCIL'
(Other definitions for stencil that I've seen before include "Copy" , "Clients (anag)" , "Sheet with holes used to make ink or paint patterns" , "Aid for drawing" , "Make patterns using thin perforated sheets" .)