Spaniard initially inspired by old measure, Picasso say (6)
I believe the answer is:
cubist
'picasso say' is the definition.
(I know that Pablo Picasso is a cubist)
'spaniard initially inspired by old measure' is the wordplay.
'initially' says to take the initial letters.
'inspired by' is an insertion indicator (some letters breathed in or 'inspired' by others).
'old measure' becomes 'cubit' (I've seen this before).
The initial letter of 'spaniard' is 's'.
's' inserted within 'cubit' is 'CUBIST'.
(Other definitions for cubist that I've seen before include "Braque, for one" , "Artist of geometrical figures" , "Painter in a style using geometric shapes" , "Braque or Picasso, for example" , "modern artist" .)