I want to keep old penny absolutely (6)
I believe the answer is:
indeed
'absolutely' is the definition.
(I've seen this before)
'i want to keep old penny' is the wordplay.
'want' becomes 'need' (synonyms).
'to keep' means one lot of letters goes inside another.
'old penny' becomes 'd'.
'need' placed around 'd' is 'ndeed'.
'i'+'ndeed'='INDEED'
(Other definitions for indeed that I've seen before include "Really - truly" , "''A friend in need is a friend . . . . . .''" , "still" , "Certainly, quite so" , "beyond doubt" .)