Yarn's second appearance by Henry (6)
I believe the answer is:
mohair
'yarn's' is the definition.
(type of yarn)
'second appearance by henry' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'appearance' becomes 'air' (both can mean an impression or character).
'by' means one lot of letters go next to another.
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'air' put after 'h' is 'hair'.
'mo'+'hair'='MOHAIR'
(Other definitions for mohair that I've seen before include "Cloth made from combings of angora goat" , "Cloth made from wool of angora goat" , "It comes from the Angora goat" , "Fine silky wool of the angora goat" , "Angora goat yarn" .)