Pa’s piano is with headless rascal (6)
I believe the answer is:
pascal
'pa's' is the definition.
(synonyms)
'piano is with headless rascal' is the wordplay.
'piano' becomes 'p'.
'is with' says to put letters next to each other.
'headless' means to remove the first letter.
'rascal' with its initial letter taken off is 'ascal'.
'p'+'ascal'='PASCAL'
(Other definitions for pascal that I've seen before include "Unit of pressure; French scientist" , "French mathematician, d. 1662 - SI unit of pressure" , "clever thinker" , "17th century French mathematician -- SI unit of pressure" , "Seventeenth century French physicist" .)