Furnish the ending with a joke (5)
I believe the answer is:
equip
'furnish' is the definition.
(equipping is a kind of furnishing)
'the ending with a joke' is the wordplay.
'ending' indicates one should take the final letters (the ending of the word).
'with' means one lot of letters go next to another.
'a joke' becomes 'quip' (a quip is a joke or witticism).
The last letter of 'the' is 'e'.
'e'+'quip'='EQUIP'
(Other definitions for equip that I've seen before include "Give kit to" , "Provide with stock" , "Furnish with requirements" , "Supply with what is needed" , "Fit out, supply" .)