Supplement dad managed to finish with little hesitation (8)
I believe the answer is:
addendum
'supplement' is the definition.
(I know that supplement can be written as addendum)
'dad managed to finish with little hesitation' is the wordplay.
'managed' indicates an anagram.
'to finish' becomes 'end' ('end' can be a synonym of 'finish').
'with' says to put letters next to each other.
'little hesitation' becomes 'um'.
'dad' with letters rearranged gives 'add'.
'add'+'end'+'um'='ADDENDUM'
(Other definitions for addendum that I've seen before include "Thing to be added" , "Extra item added at end of book" , "Dud ad-men dealt with in end of book" , "Additional item at the end of a book" , "Supplement, appendix" .)