Chap getting old penny in each helmet (9)
I believe the answer is:
headpiece
'helmet' is the definition.
(I know that headpiece is a type of helmet)
'chap getting old penny in each' is the wordplay.
'chap' becomes 'he'.
'getting' means one lot of letters go next to another.
'old penny' becomes 'd' (pre-decimal abbreviation for penny).
'in' means one lot of letters goes inside another.
'each' becomes 'apiece' (synonyms).
'd' put within 'apiece' is 'adpiece'.
'he'+'adpiece'='HEADPIECE'
(Other definitions for headpiece that I've seen before include "hat" , "Something worn on top" , "decoration on top" , "Former intellect" .)